5. GENUS — PYRGOMA.

PYRGOMA. Leach. Journal de Physique, tom. 85, 1817.

BOSCIA. Ferussac. Dict. Classique d’Hist. Naturelle, 1822.

SAVIGNIUM. Leach. Zoological Journal, vol. 2, July, 1825.

MEGATREMA. Ib. Ib.

ADNA. Ib. Ib.

DARACIA. J. E. Gray. Annals of Phil. (new series), August, 1825.

CREUSIA. De Blainville. Dict. Sc. Nat., Pl. 116, 1816-30.

NOBIA. G. B. Sowerby, junr. Conchological Manual, 1839.

The name, Nobia, is given in this work on the authority of Leach, but this must be a mistake, probably caused by some MS. name, (that fertile source of error in nomenclature), having been used.

Shell formed of a single piece; basis cup-formed, or sub-cylindrical, attached to corals.

Distribution, imbedded in corals, chiefly in the tropical seas, round the world.

 

I feel no hesitation in including the above several genera in one genus. In external appearance the P. monticulariæ (Pl. 13, fig. 5 a), which forms the genus Daracia of Gray, is the most distinct, but it is so intimately allied to the two, indeed to the three, foregoing species, that it cannot be separated from them. Of the first five species, P. grande, conjugatum, and cancellatum, form a graduated series, but with the steps very distinct, the chief difference being in the length of the spur of the tergum; for the fact of the scutum and tergum being calcified together in P. grande and conjugatum, and distinct in P. cancellatum, is certainly unimportant, as may be inferred from what we shall see in comparing together the last four species of the genus, and from what we shall see in Creusia spinulosa. The three above-mentioned graduated species are connected with the last four species of the genus, by several points of resemblance between P. grande and crenatum. The first two species, namely, P. Anglicum and Stokesii, are the most closely related together, and may indeed possibly be identical; these two, in all the characters derived from the opercular valves, resemble Balanus and other ordinary forms, and for this very reason they have some claims to be generically separated from the other species of Pyrgoma; for in these latter, the opercular valves seem to have broken loose from all law, presenting a greater diversity in character than do all the other species of Balaninæ and Chthamalinæ taken together.

General Appearance. — The shell consists of a single piece, generally without any suture, even on the internal surface; and this is the case, at least in P. Anglicum, in extremely young colourless specimens: nevertheless, in some specimens of this very species, and of P. conjugatum, there were traces of two, but only two, sutures on the sheath, one on each side towards its carinal end. The shell is much depressed or actually flat; and I have seen specimens even slightly concave; in P. Anglicum, however, the shell is steeply conical. The outline is generally oval; but in P. monticulariæ it is extremely irregular. The surface is generally furnished with more or less prominent ridges, radiating from the orifice, which is oval and small; sometimes, as in P. monticulariæ, excessively small. The colour is white, or pinkish-purple. Most of the species are small, but I have seen specimens of P. grande three quarters of an inch in diameter in the longer axis, and, including the deep, almost tubular, basis, more than three inches in length or depth.

Opercular Valves. — In three species, viz., P. conjugatum, (Pl. 12, fig. 7 c), grande, and monticulariæ, the scuta and terga, on each side, are calcified together so perfectly, that there is no trace of a suture or line of junction: in P. milleporæ, these valves are generally slightly calcified together, but with the suture distinct. The Scuta differ so much in shape in the different species, that little can be said of them in common: in P. Anglicum and Stokesii they resemble those of Balanus; but in the other species they are much more elongated than is usual, and this is carried to an extreme in the last four species; this elongation is due to a great increase in breadth, as may be inferred from the position of the apex of the valve, and from the direction of the lines of growth. But the two most remarkable characters are, first, — the extraordinary development of the adductor ridge, so that, in P. conjugatum, cancellatum (Pl. 12, fig. 5 c), grande, and crenatum, it extends considerably beneath the basal margin, being produced, in the first two species, at the rostral angle, into a point; at the tergal end of the valve, the adductor ridge, when thus much developed, blends into the articular ridge. The second very remarkable character is the addition of a special ledge along the occludent margin of the scutum, and along the carinal margin of the tergum, which I will call the occludent ledges (limbus occludens), and which serve to close the orifice leading into the sack. The occludent ledge is small in P. grande, and is clothed with thick yellow spines, giving it a brush-like appearance: in P. crenatum and dentatum it is largely developed, the ledge on the scutum being articulated with that on the tergum, as shown in Pl. 13, fig. 4 a, 4 b, — the ledges being here and elsewhere marked by little bristly points. In P. monticulariæ, however, this ledge arrives at its maximum development (Pl. 13, fig. 5 f), the rest of the valve (the scuta and terga being here, as in P. grande, calcified together) being reduced to a mere basal edge or border. Excepting for the adductor muscle, the depressions or crests for the other muscles, both on the scuta and terga, are hardly developed.

Terga: these, as in the case of the scuta, differ so much in shape in the several species, that little can be said of them in common. In P. Anglicum and Stokesii, they are of the normal shape; in P. cancellatum (Pl. 12, fig. 5 d) this, to a certain extent, is likewise the case, but the spur is produced to a quite extraordinary length. In P. grande (Pl. 13, fig. 1 b) there is no distinct spur, and the whole valve is square. In P. milleporæ (2 f) there is no spur, and the valve is arched and triangular. In P. crenatum the spur is broad, rounded, and depressed (fig. 4 b), with the carino-basal portion of the valve reduced to a mere border, barely distinguishable from the great occludent ledge. In P. monticulariæ there is no spur, and the whole valve forms a mere border to the occludent ledge; and, lastly, in P. dentatum, the valve is extremely variable in shape (fig. 3 c, 3 d, 3 f), and on its internal surface (fig. 3 g) there is an inwardly projecting, most singular and anomalous, tooth. Hence we see how wonderfully variable the terga are in this genus.

Structure of the Walls. — The shell consisting, as has been stated, of a single piece, is generally thick. From the close alliance between this genus and Creusia, it is probable that the shell, if examined immediately after the metamorphosis, would be found to show traces of being formed of four compartments. The walls are either solid or porose; their basal margin is formed by strong crenated ridges, answering to the longitudinal septa in Balanus; but these in P. monticulariæ are modified into a very irregular surface. The internal surface of the shell is generally smooth, or only slightly ribbed. The sheath has its lower edge free in P. Anglicum and Stokesii, and in a slight degree in P. milleporæ, but in the other species it is closely attached to the walls. In P. monticulariæ the sheath might easily be overlooked. In P. Anglicum, grande, and crenatum, it descends almost to the basal margin of the depressed shell, and as the opercular valves and membrane are attached to the lower edge of the sheath, the animal’s body necessarily comes to be almost exclusively lodged in the cup-formed basis. In P. grande and conjugatum, the lines of growth in the sheath are bent upwards on each side, at points corresponding with the line of union between the scutum and tergum, in a manner I have not seen in any other cirripede; and this sometimes gives the appearance of two lateral sutures. I may here remark, that the manner of growth in Pyrgoma is almost the converse of that in Balanus, Tetraclita, and other allied genera; for in these latter, the basis increases in diameter, and the shell chiefly in height; whilst in Pyrgoma, the shell, from being so flat, increases almost exclusively in diameter, whereas it is the basal cup which is added to in height or depth.

Basis. — This in all the species is more or less regularly cup-formed or sub-cylindrical. In P. grande it penetrates the coral to a surprising depth; in P. monticulariæ it is irregular in outline, corresponding with the shell. The basis is generally almost wholly imbedded in the coral; but this is not the case with P. Anglicum, in which the basis is generally exserted, as it is in a slight degree in P. grande. The shelly layer forming the basis, in most of the species, is very thin, and is finely plicated owing to its edge folding between the ridges or septa that form the basal edges of the shell; this is very conspicuous in P. cancellatum. The basis is not permeated by pores, except in P. Anglicum. In some sessile cirripedes a cleft, covered only by membrane, may be observed all round between the lower edge of the shell and the basis; a cleft of this nature is rather conspicuous in P. crenatum, so that small portions of the septa on the internal surface of the walls can be seen from the outside. In P. monticulariæ, an analogous structure, developed to an extreme degree, presents a very different and unique appearance; the shell is nearly flat, and the smooth outer lamina does not nearly reach to the circumference, a wide border being thus left exposed, which is roughened (Pl. 13, fig. 5 a, 5 c, 5 d, 5 e) by the exposure of the irregular septa. I have not seen a fresh specimen, but there cannot be a doubt that this border is properly covered by membrane.

Animal’s Body. — From some cause, perhaps from the corals in which the species of Pyrgoma are imbedded, long remaining damp, the internal organs are generally badly preserved. I have received, in spirits of wine, only P. Anglicum, but I have examined dry specimens, in a tolerable condition, of P. milleporæ and crenatum. As neither the mouth nor cirri, in these three species, offer any noticeable characters, distinct from those in Balanus or Acasta, my ignorance of these organs in the other six species is not important. In the above three species the labrum is deeply notched, with about three teeth on each side of the notch, except in P. milleporæ, in which the number is six. In all, the mandibles have five teeth, the two lower ones being small: the maxillæ are not notched: the outer maxillæ are bilobed. In the Cirri, the rami in the first pair are very unequal in length, the segments being slightly protuberant in the shorter ramus. On the segments of the posterior cirri there are four pairs of spines in P. Anglicum, and three pairs (of which the second and third are short) in P. milleporæ. At the dorsal basis of the penis there is a small straight projecting point.

Affinities. — The species (with the exception of the first two) are much more distinct from each other, and more easily determined than is usual with sessile cirripedes; it is, however, quite useless to attempt naming the species without disarticulating and cleaning the opercular valves. Although these valves differ so greatly in some of the species from those of Balanus and the allied forms, the genus itself, as a whole, does not differ much, except in the shell not being divided into compartments, and in the basis being cup-formed and not generally permeated by pores, — these latter characters being in common with Acasta. With respect to the absence of separate compartments, it should be remembered, that in the same species of Tetraclita we have individuals with the four compartments distinct and furnished with radii, and other individuals without any trace of a suture externally, — the outer lamina of shell (though not the inner) having become completely confluent all round. At the commencement of this description, when giving my reasons for uniting the several proposed genera into one genus, I gave a sketch of the affinities of the species: I have only to add, that the following sub-genus Creusia is closely, perhaps too closely, allied to Pyrgoma.

Geographical and Geological Distribution. — Most of the species are inhabitants of the hot coral-growing zones, in both the eastern and western hemispheres, but more especially, as it would appear, in the East Indian Archipelago. From the habits of the corals, most of the species must be inhabitants of shallow water. Pyrgoma Anglicum, however, is an inhabitant of deep water on the southern shores of England, whereas at St. Jago, in the Cape de Verde Islands, I myself collected it adhering to a Caryophyllia, within the tidal limits. This same species existed on the shores of England during the Coralline Crag period; and at this epoch it attained a larger size than at present. Two species of the genus, according to Sismonda, are found in the tertiary beds of Piedmont. Mr. Stutchbury, who is so well acquainted with recent cirripedes, informs me that he has for many years examined fossil Secondary corals, in the expectation of finding imbedded, species of this genus or of some allied form, but without success. The same species of Pyrgoma is by no means always confined to the same coral: I have seen P. crenatum on four or five different corals, and P. Anglicum on at least three kinds: on the other hand, I have seen P. milleporæ only on the Millepora complanata (a member, as I believe, of the vegetable kingdom), and P. monticulariæ on a Monticularia from near Singapore.

 

1. PYRGOMA ANGLICUM. Pl. 12, fig. 4 a-4 c.

PYRGOMA ANGLICA. G. B. Sowerby. (sine descript.) Genera of Recent and Fossil Shells, fig. 7, No. 18, Sept. 1823.

MEGATREMA (ADNA) ANGLICA. J. E. Gray. Annals of Philosoph. (new series), vol. x, Aug. 1825.

PYRGOMA SULCATUM. Philippi. Enumeratio Molluscorum Siciliæ, Tab. 12, fig. 24, (1836).

  ——  — ANGLICA. Brown. Illustrations of Conchology, (2d edit., 1844), Tab. 53, fig. 27-29.

Shell steeply conical, purplish red: orifice oval, narrow: basis permeated by pores, generally exserted out of the coral: scutum and tergum sub-triangular.

Hab. — South coast of England and of Ireland, (12 to 45 fathoms, Forbes and MacAndrew); Sicily; Madeira; St. Jago, Cape de Verde Islands; generally attached to the edge of the cup of a Caryophyllia, in deep water, but at St. Jago within the tidal limits; Mus. Brit., Cuming, Lowe, &c.

Fossil in the Coralline Crag, Ramsholt; Mus. S. Wood.

General Appearance. — Shell steeply conical, slightly compressed, the lower part with rounded, approximate, radiating ribs: colour dull purplish-red: orifice oval, small, and narrow. The basis is not deeply conical, and occasionally is even flat. Generally it stands exserted; but in the Coralline Crag specimens, it is almost wholly imbedded. Externally it is furnished with ribs corresponding with those on the shell. The largest recent specimen which I have seen, from St. Jago, was .22 of an inch in basal diameter; but some few of the British specimens are nearly as large, and one of the fossils from the Coralline Crag a little larger.

The Scuta and Terga are of the ordinary shape of these valves in Balanus and its allies. Scuta triangular, with the basal margin a little curved and protuberant: adductor and articular ridges distinct from each other, moderately prominent: there is a small hollow for the lateral depressor muscle. Terga triangular, with the spur rather narrow, moderately long, placed near, but not confluent with the basi-scutal angle of the valve. The basal margin forms an angle rather above a right angle with the spur. Internally, the articular ridge and crests for the depressor muscles, feebly developed.

Internal Structure of the Shell and Basis. — Internally, the shell is ribbed more or less prominently. The lower edge of the sheath, which is reddish, and extends far down the walls, seems always to project freely. In several specimens there were on each side, at the carinal end of the shell, a trace of a suture, which could be perceived only on the sheath. The basis appears always to be permeated by minute tubes or pores, though these are sometimes rather difficult to be seen.

 

2. PYRGOMA STOKESII. Pl. 12, fig. 6.

MEGATREMA STOKESII. J. E. Gray. (sine descript. aut figurâ) Annals of Philosophy, (new series), vol. 10, Aug. 1825.

Shell moderately conical, pale-purplish red; orifice oval: basis not permeated by pores, deeply imbedded in the coral: scutum and tergum sub-triangular.

Hab. — Imbedded in the Mycedia (Agaricia) agaricites; therefore from the West Indies; Brit. Mus. and Stutchbury.

I am greatly indebted to Mr. Dana for having named for me the coral in which this species was imbedded, and informing me that it is a West Indian species.

 

This species comes so close to the last, that I am not sure that I have acted rightly in retaining it, but I think that it is distinct; and in this case, it is the representative, on the other side of the Atlantic, of P. Anglicum of our own side. It will be sufficient to point out the few points of difference. The shell is much more depressed, with the orifice oval, larger, and not so narrow. It is apparently of a paler red, and the radiating ribs perhaps not so prominent. The basis offers the most important difference, being deeply imbedded in the coral; and there is not the least appearance of the thin shelly layer, of which it is composed, being permeated by pores, as, we have seen, is always the case with P. Anglicum. As in this latter species, the sheath here depends freely. The opercular valves are closely similar; but in the scutum, the adductor ridge occupies a rather more central position; and in the tergum, the basal margin is more inclined towards, or forms a greater angle with, the spur: these differences, by themselves, I consider quite insufficient to characterise a species; but conjoined with the flatter shell, the larger orifice, the more deeply imbedded and non-porose basis, they may, I think, be admitted as specific. In dimensions, this species seems to attain a slightly larger size than P. Anglicum, for several specimens were .22 of an inch in diameter.

 

3. PYRGOMA CANCELLATUM. Pl. 12, fig. 5 a-5 f.

PYRGOMA CANCELLATUM. Leach (!). Encyclop. Brit., Supplement, vol. 3, Pl. 57, 1824.

  ——  — LOBATA. J. E. Gray (!). Annals of Philosophy, (new series), vol. 10, 1825.

CREUSIA RAYONNANTE. De Blainville. Dict. Sc. Nat. (sine descript.), Pl. 116, fig. 7, a, b.

Shell with the circumference generally lobed: scutum elongated, with the adductor ridge descending far below the basal margin, and produced at the rostral end into a square point; tergum with the spur four times as long as the upper part of the valve.

 

Hab. — Imbedded in a Gemmipora, probably from the West Indies; Mus. Brit. and Cuming.

In this case I am again indebted to Mr. Dana for naming the coral; he informs me that the genus is found in the Pacific and East Indies; the specimen sent he believes is the G. cinerascens.

Appearance and Structure of Shell. — Shell nearly flat, sometimes tinted dull dirty purple, with the surface marked by slight, broad, approximate ridges, the ends of which form considerable projections, giving to the shell, when not too much encrusted by coral, a lobed border. In young specimens (.15 of an inch in diameter) some of the points projected half as much as the semi-diameter of the shell, giving it a radiating appearance. The orifice is oval and rather small, but of variable size. The shell is thick, and, near the outer lamina, is penetrated by pores: the internal surface is smooth. The sheath (fig. 5 b) extends down but a short distance from the orifice; it is closely attached to the walls: the lines of growth, at a point on each side, bend a little downwards (instead of upwards, as in the two following species), and hence the lower edge of the sheath irregularly projects downwards on each side. The basal cup, internally, is plicated, the hollows corresponding with the projecting, longitudinal, parietal septa, which form the lobed border of the shell. The largest specimen which I have seen, was rather above .4 of an inch in diameter.

The Scuta and Terga are not calcified together: they are both much elongated. Scutum (fig. 5 c, 5 e). — For the first time in the genus, or indeed in the family, this valve presents a remarkable character in the adductor ridge being immensely developed, so as to project far below the ordinary basal margin. At the rostral end, it at first appears to project even more than it really does, for the toothed occludent margin is in fact a prolongation of the true valve, as distinct from the adductor plate. Excluding this very narrow, prolonged, occludent margin, the adductor plate projects for a length equalling the rest of the valve. Along the tergal margin, the adductor plate is united to the articular ridge; and at the rostral end, it is produced into a square tooth, whence a square-edged ridge extends on the surface of the plate upwards to the ordinary basal margin of the valve. The exact shape of this adductor plate varies a little, as does the degree to which it is closely attached to the ordinary basal edge of the valve. The valve, as distinct from the adductor plate, is narrow, with the basal margin regularly curved. The articular ridge is very prominent.

Terga extremely narrow, linear, consisting in chief part of the spur, which is fully four fifths of the entire length. Externally (fig. 5 d) the valve is furrowed, with the edges more or less folded in along the spur. The upper or ordinary part of the valve is about one third wider than the spur. The basi-carinal angle is sharp, owing to the basal margin being a little hollowed out. A special plate of shell (fig. 5 f), hollow under its basal edge, runs from the carinal margin to the articular ridge, which latter is situated in the middle of the valve, and against which the articular ridge of the scutum abuts. The spur is central; its end is bluntly pointed. The total length of the tergum rather exceeds that of the scutum, the produced adductor ridge being included in the latter.

Affinities. — Under P. grande I shall make a few remarks, showing that in several characters P. cancellatum and grande are at opposite ends of a short series, with P. conjugatum intermediate between them.

 

4. PYRGOMA CONJUGATUM. Pl. 12, fig. 7 a-7 c.

Dr. Gray thinks this is the Pyrgoma stellata, of Chenu, (‘Illust. Conch.’); it may be so; but the figure given of the shell will do equally well or rather better for the Pyrgomum dentatum of this work, and for some varieties of P. crenatum. Without a careful description of the opercular valves, it is really impossible to recognise, with any approach to certainty, the species of this genus.

Shell nearly flat with approximate radiating ridges: scutum and tergum calcified together without any suture: scutum with the adductor ridge descending below the basal margin, and produced at the rostral end into a point: tergum with the spur about as large as the upper part of valve.

Hab. — Red Sea; Brit. Mus. and Cuming.

Appearance and Structure of Shell. — Shell white, or with a tinge of purple; nearly flat, with moderately prominent, narrow, approximate ridges, radiating from the orifice, which is oval, rather narrow, and not very small. In the largest specimens the ridges are less prominent than in the figure given. The walls are thick, and not at all porose: the sheath extends down almost to the base of the shell, and its lower edge is closely attached to the walls: on each side, towards the carina, there is a trace of a suture, and the lines of growth on the sheath are here a little upturned. The basis is deeply imbedded and internally furrowed; the calcareous layer forming it, is thin. The largest specimen was .4 of an inch in the longer diameter.

The Scuta and Terga (fig. 7 b, 7 c), in all the specimens which I have seen, are calcified together, with no trace of a suture. There is, however, a slight furrow, which, I believe, marks the normal line of separation between the two valves; and in the following description this is assumed to be the case. The Scutum has the adductor ridge greatly developed, so as to project below the ordinary basal margin to a distance as great as the height of the valve. At the rostral end, this adductor ridge or plate is produced into a point; and at the tergal end, it is blended with the articular ridge, and united to the inner face of the tergum. That portion of the scutum which corresponds with the valve in ordinary cases, and alone is externally visible as long as the operculum is united by the opercular membrane to the sheath, is narrow, with the basal margin considerably hollowed out: the occludent edge is formed into thick teeth. The Tergum is elongated, rather exceeding in length the scutum, the latter being measured from the apex to the rostral projection of the adductor plate. The surface of the valve is depressed in the line of the spur, with the basal end of the latter bluntly pointed. A very slight flexure (fig. 7 c) on the basal margin indicates where we may believe the spur to commence, showing that it rather exceeds in length the whole upper part of the valve. The lines of growth obscurely indicate a tendency to the formation of a slight “occludent ledge” along the carinal margin. Traces are just visible of crests for the attachment of the tergal depressor muscles.

 

5. PYRGOMA GRANDE. Pl. 13, fig. 1 a-1 d.

NOBIA GRANDIS. G. B. Sowerby, junr. (sine descript.) Conchological Manual, fig. 29, 1839.

CREUSIA GRANDIS. Chenu. Illust. Conch., Tab. 1, fig. 2 a, sed non, fig. 2.

It is quite possible that this may be the Balanus duploconus of Lamarck, but with such a character as the following, who can recognise a species? “B. testæ parte supremâ univalvi, indivisâ, convexâ: inferiore turbinatâ, non clausâ: aperturâ ellipticâ. L’exemplaire est sans opercule et incomplet.”

Shell moderately convex, nearly smooth: scutum and tergum calcified together without any suture: scutum furnished with a small occludent ledge, with the adductor ridge descending below the basal margin: tergum square without a spur.

Hab. — Singapore and East Indian Archipelago; Mus. Brit., Cuming, Stutchbury; imbedded in two kinds of coral.

Appearance and Structure of Shell and Basis. — The shell is conical, though to a variable degree, and sometimes is much depressed. The surface is smooth, with only traces of narrow approximate ridges. The colour is white, often with a tinge of dark purple. The orifice is oval, and moderately large. The shell and a small portion of the basis usually stand exserted above the coral. The walls are of variable thickness; when thick, the pores, by which they are permeated, are but little apparent; sometimes there is more than a single row of pores. The points of the septa on the basal edge of the shell are small. The internal surface of the shell is smooth. The sheath is closely attached to the walls, and descends nearly to the basis; on each side its lines of growth are slightly upturned. The basis is deeply cup-formed or cylindrical, and in section oval like the shell; it penetrates the coral to a very remarkable depth, — in one instance to three inches. The shelly layer forming it, is thin, finely furrowed, and not permeated by pores. This is the largest species in the genus; one specimen was three quarters of an inch in its longer diameter, and above three inches in length.

The Scuta and Terga are calcified together, without any trace of a suture; the line of junction can be inferred only from the analogy of P. conjugatum, in which species the valves have a more normal character, and are separated by a slight furrow. It may be seen in the figure (Pl. 13, fig. 1 d) of the right and left hand opercular valves, viewed from vertically above in their proper relative positions, how abnormal their appearance is, which is partly caused by the spinose occludent ledges, presently to be described, but chiefly from the carinal margins of the two terga not being straight and parallel, as in all other cirripedes, and therefore not meeting each other, as is usual. In other genera, only the upper part of the carinal margin of the two terga can be opened for the exsertion of the cirri, the lower portion being united by membrane; but here, I have little doubt, from the position of attachment of the adductor muscle (fig. 1 c), so close to and almost on the terga, that the whole length of the carinal margin of the two terga is free or disunited for the protrusion of the cirri. This opening between the two terga evidently cannot be closed, but is probably filled up, and the animal thus protected, by the dorsal surfaces of the curled-up cirri; such, I believe, being likewise the case with some pedunculated cirripedes, as with Conchoderma. The scutum has a large adductor plate, which extends some little way (namely, about one quarter of the height of the valve), below the ordinary basal margin. This latter margin is slightly sinuous, and a little hollowed out towards the tergal corner of the valve. I believe that the ridge, which runs down to the basi-scutal corner of the tergum, though appearing to be part of the scutum, really belongs to the tergum. The adductor scutorum plate is not, as in the last two species, produced into a point at the rostral angle; at the tergal end it blends into the under surface of the tergum. The occludent margin is coarsely toothed. Rather on the under side of this margin and in the upper part, there is a narrow occludent ledge, which extends up beyond the apex of the valve, and thence runs a little way along the carinal margin of the tergum. This ledge is thickly clothed with strong, yellowish-brown spines, and hence appears like a brush. It is remarkable that the cavity for the adductor scutorum muscle is situated almost on the tergum.

The tergum is of large size, and nearly square; it is, in appearance, separated from the scutum by a ridge running up to the apex. The basal margin forms a right angle with the carinal margin, along which latter margin the lines of growth are upturned, and blend into the occludent ledge, which is common to the two valves. There cannot be said to exist any spur, the whole basal margin being almost straight; nevertheless, on close examination, the ridge which in appearance separate the scutum and tergum, may, I think, be safely considered as one side of the spur (which, it should be remembered, has in all ordinary cases a longitudinal furrow or depression), and the other side of the spur is, apparently, very feebly indicated by a slight flexure in the middle of the basal margin. Hence, if the spur had been developed, it would probably have been half as wide as the valve. There are no crests for the tergal depressor muscles.

Affinities. — The present species, with the last two, form an interesting series. Pyrgoma grande and conjugatum, however, are more closely allied to each other than to P. cancellatum. In the scutum, the whole valve is least elongated, with the adductor plate least developed, in P. grande, and most elongated, with the adductor plate most developed, in P. cancellatum. In the outline of the tergum the range of shape is quite remarkable; in P. conjugatum, which stands between the other two species, the spur is rather long, whereas in P. grande there is no spur at all — a very unusual circumstance — and in P. cancellatum, at the other end of this short series, the spur attains a length wholly unparalleled in any other cirripede.

 

6. PYRGOMA MILLEPORÆ. Pl. 13, fig. 2 a-2 f.

From external aspect I suspect this species to be the Creusia madreporarum, Leach (?), as given in Chenu, ‘Illust. Conch.,’ Tab. 1, fig. 6. But I feel sure that Leach has nowhere published this name; and it may be observed that Chenu gives it with a mark of doubt. The shell in its imbedded state is only figured; the opercular valves are not given; and no descriptive details are added. Under these circumstances I have not adopted this name; I have, perhaps, been in some degree influenced by the fact that this species, judging from the many specimens examined by me, is never imbedded in madrepores, but exclusively in millepores, so that Chenu or Leach’s specific name of Madreporarum is singularly inappropriate.

Shell with the orifice narrowly ovate: sheath dark purple: Scutum much elongated: tergum triangular, convex, without a spur.

Hab. — Philippine Archipelago (Mindoro Island), Mus. Cuming. Mus. Brit., Stutchbury, &c. Imbedded in Millepora complanata, sometimes associated with Balanus Ajax.

Appearance and Structure of Shell. — Shell oval, flat, coloured pale dull purple, or white, with slight and narrow ridges, radiating, from the orifice, which is not quite central, but is placed rather nearer to the carinal than to the rostral end of the shell. The orifice is small and narrow; the carinal end being rounded, and the rostral end narrower and sharper, — this being the exact reverse of the usual shape of the orifice in the Balanidæ. The walls are thick, and are formed of large square tubes. The internal surface of the shell is smooth. The sheath (fig. 2 b) is much more elongated than the shell, for at the rostral end it extends to the basal margin, and at the carinal end nearly to it, whereas on the two sides it is some way distant from the base. The orifice is considerably out of the centre of the sheath, being placed nearest to the carinal end. The sheath has its lower edge slightly prominent or free; the lines of growth are neither turned up nor down on the two sides, as in the last three species. When nearly full grown the sheath is coloured dark purple, but when young it is white, hence the upper part is white, surrounded by an oblong purple ring, and this is surrounded by the white shell. The basal cup is deep, and internally nearly smooth. The largest specimens were .3 of an inch in their longer diameter. Great masses of the Millepora are absolutely studded with this Pyrgoma, with usually more specimens on one side of the plate or branch than on the other. They stand in approximately parallel positions, the broad or carinal ends of the orifices pointing upwards.

Scuta. — The scuta and terga are closely united, and are often (perhaps always) in some slight, though variable degree, calcified together; and hence they often break, rather than separate, at the line of articulation. The external fissure or line of junction between them (fig. 2 c) is oblique to the longer axis of the scutum; in the uppermost part of the valve it is sometimes almost obliterated. The two valves together are nearly as long as the sheath, and consequently much longer than the orifice of the shell. The scutum is much elongated, being fully four times as broad as high. The valve narrows towards the rostral end, but in a variable degree: the basal margin is hollowed out a little (but to a variable amount), close to the basi-tergal corner. Along the occludent margin a slip of the valve, widening downwards, is a little bent inwards, and this inflected portion is separated from the rest of the surface by a slightly angular ridge, running from the apex to near the rostral angle. Internally, at the basi-tergal corner, a slight ridge, parallel and close to the basal margin (and which can be seen only when the basal edge of the valve is held upwards), represents the adductor ridge, which we have seen so largely developed in the last three species, and shall again see in P. crenatum. The articular ridge (fig. 2 d) is extremely prominent, consisting of a more or less rectangular shoulder.

Terga: these are rather small compared to the scuta: they are triangular and much arched: there is no trace of a spur. Internally (fig. 2 f), the articular ridge is central: there are some vestiges of crests for the depressor muscle.

Affinities. — This well-marked species, in the tendency of the opercular valves to be soldered together, and in the remarkable absence of a trace of a spur to the tergum, is allied to P. grande, but it is more closely allied to the three following species.

 

 

7. PYRGOMA DENTATUM. Pl. 13, fig. 3 a-3 g.

Scutum much elongated, with a tooth-like articular projection: tergum convex, irregularly triangular, sometimes with an imperfect spur, and on the internal surface with an inwardly projecting tooth; scutum and tergum furnished with an occludent ledge.

Var. (1), 3 c, 3 g: tergum, with a sharp internal tooth, projecting rectangularly inwards.

Var. (2), 3 d: tergum, with a broad blunt internal tooth, depending beneath the spur-like portion of the valve.

Var. (3), 3 f: tergum, with the basi-carinal end of the valve truncated, with a small blunt internal tooth projecting rectangularly inwards.

Hab. — Red Sea; Mus. Brit. and Cuming. Also associated with Pyrgoma crenatum, and attached to Meandrina spongiosa.

Mr. Dana informs me that he believes that this coral comes from the West Indies; though the specimens originally described by him had no label. If this be so, both Pyrgoma dentatum and crenatum have very wide ranges.

Appearance and Structure of Shell. — Shell nearly flat, oval, white or pink, with rather distant prominent ridges radiating from the moderately large (for the genus) oval orifice. The ridges are often obscured, and apparently sometimes almost obliterated by the encrusting coral. Shell permeated near the outer lamina by short imperfect pores: internal surface smooth: sheath inconspicuous, descending rather more than half way down the walls; lower edge closely attached to the walls. Basis deep. Diameter of largest specimen .3 of an inch.

Scuta: these are elongated, but to a very variable degree, some specimens being quite three times, and some barely twice as broad as high. I observed this same variability in two sets of specimens, differing, as we shall presently see, in the form of their terga: it depends in part, but not wholly, on the varying width of the occludent ledge, which is sometimes only a fourth, and sometimes half as high as the rest of the valve. This is the first species in the genus in which the occludent ledge — a structure peculiar to the genus — has been amply developed. The basal margin of the valve is slightly sinuous, and a very little hollowed out near the basi-tergal corner; it is also very slightly reflexed, the reflexed portion being separated from the upper part of the valve by a very slight depression or even furrow. I notice this slightly reflexed portion, simply as indicating a well marked feature in the basal margin of the following closely allied species. Internally (3 g), the adductor ridge is thick and slightly prominent, but does not descend beneath the basal margin: it blends into the articular ridge, which here projects in a remarkable manner and degree (3 b, 3 e, 3 g), like a rounded tooth. This tooth is in part a development of the occludent ledge; it varies much in shape. The line of junction between the scutum and tergum is nearly straight, and nearly at right angles to their longer axes. In some specimens the scuta and terga are partially calcified together.

Terga: in three sets of specimens the terga differed considerably, but as in every other respect there was the closest resemblance, I do not doubt that these are merely varieties. In all three, the valve is rather small, irregularly sub-triangular in shape, and externally somewhat convex; in all three, there is an occludent ledge, of variable width as in the scuta; and in all three, there is an internal tooth-like projection, of variable form, unlike anything I have seen in any other cirripede. In the first variety (Pl. 13, fig. 3 c, 3 g), the basi-carinal corner of the valve is bluntly pointed, and a slight flexure separates this portion of the valve from the other and scutal half, which latter thus exhibits some tendency to be converted into a spur: on the internal surface (3 g) of this spur-like portion of the valve, there is a rather long, sharp tooth, which projects rectangularly inwards; it is flattened in a plane at right angles to the longer axis of the scutum and tergum together: it cannot be seen from the outside. In the second variety, the shape of the valve is not very different (Pl. 13, fig. 3d), excepting that the flexure, separating the basi-carinal corner of the valve from the spur, is deeper; but on the internal face of the spur, the tooth is far broader than in the first variety, and is flattened quite differently, viz., in a plane nearly parallel to the surface of the valve, and instead of projecting rectangularly inwards, it depends beneath the basal edge of the so-called spur, and can be seen from the outside. In the third variety (fig. 3 f), the whole carinal end of the valve is cut off, and there can hardly be said to be any trace of a spur, yet a slight furrow apparently marks the line of separation between the basi-carinal portion of the valve, here become very narrow, and the broad, irregular part, which would have formed the spur had such been developed: on the internal surface of the latter portion of the valve, there is a very small, blunt, slightly flattened tooth, projecting inwards, and more resembling that in the first than that in the second variety.

Affinities: under P. crenatum I will point out the diagnosis and relationship between this and that species.

 

8. PYRGOMA CRENATUM. Pl. 13, fig. 4 a, 4 b.

PYRGOMA CRENATUM. G. B. Sowerby. Genera of Recent and Fossil Shells, (No. 218, Sept. 1823), fig. 1 to 6.

Scutum much elongated, with the adductor ridge descending below the reflexed basal margin: tergum with a broad depressed spur: scutum and tergum furnished with a wide occludent ledge.

Hab. — Philippine Archipelago; Singapore; Mus. Brit., Cuming, Stutchbury; sometimes associated with Creusia spinulosa.

Appearance and Structure of Shell. — Shell not distinguishable from that of P. dentatum; nearly flat, oval, white, sometimes pale pink, with rather distant prominent ridges radiating from the moderately large oval orifice. Shell solid, or permeated near the outer lamina by short imperfect pores: internal surface smooth. The sheath descends nearly to the base of the walls; it is but little conspicuous, and its lower edge is closely attached to the inner surface of the shell. Basis deep. Diameter of largest specimen under .3 of an inch.

Scuta: in this species the scutum is more abnormal than in any other Cirripede in the whole family: this is owing both to the adductor ridge descending far beneath the basal margin, and to the great development of the occludent ledge; hence the middle and very narrow portion of the valve alone answers to the scutum, as seen in other genera. The whole valve, including the adductor ridge and the occludent ledge, is narrow, being more than twice as broad as high; but the proportional width varies, owing chiefly (as in the last species) to the varying width of the occludent ledge. This ledge commences a little way from the rostral point of the valve, and gradually widening, extends to the apex, where it is either as high or twice as high as the rest of the valve. It is articulated by a convex surface, and by a hollow on its under side with the occludent ledge of tergum. The scutum cannot be said to have any tergal margin; without, indeed, the articular surface of the occludent ledge be thus called. The basal margin is curved, and considerably reflexed, of which peculiarity we have seen a vestige in the last species: the reflexion is not well shown, owing to the foreshortening of the reflexed edge, in fig. 4 a: this reflexed edge not being shown, causes the lines of growth to appear as if they ran more transversely to the longer axis of the valve, than they really do; for they run nearly as in the scutum (fig. 3 e) of P. dentatum. The direction of these lines of growth is of importance, for they show that the elongation of the scuta is due to an inordinate increase in their breadth, as compared to the same valves in ordinary species. The adductor ridge, having a sinuous margin, runs from near the apex to near the rostral angle: it descends below the basal margin about as far as the height of the true valve, excluding the occludent ledge. Of course this adductor ridge or plate lies beneath the membrane connecting the opercular valves with the sheath, and is concealed by it, as long as these valves remain within the shell. The edge of the occludent ledge is straight, but the edge of the middle portion of the valve, that is of the true valve, is much bowed.

Terga (fig. 4 b): these are of so irregular a shape that they can hardly be described; they may, however, be said to consist of two portions joined together, of which the lower portion is a little elongated transversely, of somewhat variable shape, with part of its surface considerably depressed (compared with the rest of the valve), sometimes being even concave: this concave portion apparently answers to the spur in other cirripedes. At the scutal corner of the valve there is a shoulder (perhaps answering to the inflected scutal margin in an ordinary tergum), which locks into a hollow on the under side of the occludent ledge of the scutum. The internal surface of the so-called spur is rounded and convex. The upper part of the tergum is in main part formed by a great occludent ledge; but this, on its lower side, is bordered by a narrow irregular slip, which, as shown by the lines of growth, represents the whole of the ordinary valve, excepting, of course, the spur already described. The occludent ledges of both valves support some fine spines.

Affinities. — Observing how extraordinarily the terga varied in P. dentatum, and that the shells were identical in that and the present species, it occurred to me at first that they might, perhaps, be both extreme varieties of one form: but in the scutum of P. crenatum, the invariably great development of the adductor plate, — the marked manner in which its basal margin is reflexed, — the absence of a tooth-like articular projection, — and again in the tergum of P. crenatum, the invariably large size of the concave spur, without any internal tooth, altogether convince me that the two species must be considered as distinct. This species is allied to P. grande, in the scutum of that species having an occludent ledge, though small, and a great adductor plate. I have only further to remark, that the figure of the opercular valves, given in Sowerby’s Genera of Recent and Fossil Shells, is so good, that there can not be the least doubt about the present identification.

 

9. PYRGOMA MONTICULARIÆ. Pl. 13, fig. 5 a-5 f.

PYRGOMA (DARACIA) MONTICULARIÆ. J. E. Gray (!). Zoological Miscellany, , 1831.

Shell of an irregular shape, with a roughened exterior border: orifice minute, circular: scutum and tergum both much elongated, calcified together without any suture, both furnished with a broad occludent ledge.

Hab. — Singapore; Mus. Brit., Cuming, and Stutchbury. Sometimes associated with Creusia spinulosa.

Appearance and Structure of Shell. — Shell dull white, very irregular in outline, sometimes rounded, more often unequally elongated, and frequently star-shaped, — the projections being quite irregular. Whole shell nearly flat, but with the central part saddle-backed, or formed into a more or less prominent ridge, extending in the line of the longitudinal axis of the animal’s body: the circumferential portions of the shell not unfrequently are a little recurved upwards. Orifice extremely minute, circular. The outer lamina of shell, which is smooth, does not extend to the circumference, and consequently a rather broad, nearly equal border, which is rough, surrounds the whole shell. I have no doubt that, when the shell was alive, this border was covered by a membrane, which, in drying, has curled up and been lost, in the same manner as the strictly analogous but narrow open seam between the basal edges of the shell and the basis in some cirripedes (as in the last two species of Pyrgoma) is protected. The roughened border can sometimes be plainly seen to be formed of normal (Pl. 13, fig. 5 e) longitudinal septa having crenated edges, with shorter septa between the longer ones; but more often the septa are so irregular, and so much branched (5 d), that the whole resembles a mass of moss. Why the outer lamina of the shell in this one species does not nearly reach the circumference of the walls, I cannot conjecture. The extremely irregular, depressed shape of the shell, with the minute circular orifice, and the singular rough circumferential and often slightly reflexed border, together give to this species so peculiar an aspect, that until close examination I did not believe that it was a cirripede. The extreme irregularity of shape depends in great part upon the irregular growth of the Monticularia, in which it is imbedded.

Internally (fig. 5 b) the walls are smooth, but they are perforated by many quite irregular, small orifices, which have the appearance of having been formed by some boring animal, but really serve, as I believe, to admit threads of corium into certain irregular pores which penetrate the shell. The sheath descends but a very short distance from the orifice: it is closely attached to the walls, and might easily be overlooked. The basis is deep, of an irregular outline, like that of the shell, and is formed by a very thin shelly layer. The largest specimen which I have seen, measuring from the extreme projecting points, was .4 of an inch in diameter.

The Scuta and Terga (fig. 5 f) are calcified together without any trace of a suture; together they form a bow with the two ends curled rather abruptly inwards; they are both extremely narrow, but furnished with an occludent ledge, twice or thrice as high as the proper valves themselves. This occludent ledge, which is finely hirsute, begins at about one third of the length of the scutum from the rostral angle, and runs to near the basi-carinal angle of the tergum. The scutum itself is curved, with a slip, along the true occludent margin (best seen at the rostral end), lying in a different plane from the rest of the valve, much in the same way as in the scutum of P. milleporæ. The adductor ridge descends a very little below the basal margin of the valve, and extends for nearly its entire length: this adductor ridge makes the proper valve even narrower than it at first appears. The Tergum is extremely narrow, forming merely a border to the occludent ledge; but it is not short, being about two thirds of the length of the scutum. There is no trace of a spur; indeed, the valve is rather narrower where the spur should have stood, than it is at the basi-carinal end. The scuta and terga are calcified together by their apices.

Affinities. — Although this species, as above stated, differs so remarkably in external appearance from the other species of the genus, and, indeed, of the whole family, yet the shell in no one essential point of structure materially differs from its congeners; and if we compare the opercular valves with those of the three last species, we shall be struck with their close, yet graduated, affinity: in P. milleporæ the scuta and terga tend to become calcified together, and the rostro-occludent end of the scutum is bent into a different plane from the rest of the valve. In P. dentatum we have an occludent ledge exactly as in the present species; but in that species the adductor plate is less developed than in P. monticulariæ; on the other hand, in P. crenatum, the adductor plate is more developed than in P. monticulariæ. If in P. crenatum we were to remove the spur from the tergum (and it is much less developed in P. dentatum; and in P. milleporæ it is entirely absent) this valve would be almost identical with that of P. monticulariæ. Under these circumstances I consider it impossible to separate the present species as a distinct genus.

 

Species Dubiæ.

The Daracia Linnæi of J. E. Gray (Annals of Philosophy, new series, vol. 10, 1825), was published without description or figure.

The Megatrema semicostata of G. B. Sowerby, junr. (Conch. Manual, fig. 33, 1839), is not described, and is very indifferently figured without the opercular valves, and therefore can never be recognised.

There is an admirable figure of a Pyrgoma, without any specific name, in the great ‘Description d’Egypte,’ but from the want of some details, I cannot positively recognise the species; I am inclined to believe that it is the P. dentatum of this work.

M. Chenu, in his grand Illustrations Conchyliologiques, has given most beautiful figures of several species of Pyrgoma, and of Creusia, but unfortunately, from the opercular valves not having been figured, I find it impossible to recognise them. The new species are Pyrgoma stellata (on which I have appended a note under P. conjugatum), and P. spongiarum and P. corymbosa of Valenciennes.