CREUSIA. Leach. Journal de Physique, tom. 85, July, 1817.
Compartments four, furnished with radii; basis cup-formed: attached to corals.
Distribution, imbedded in corals throughout the tropical seas.
Creusia is closely allied to Pyrgoma; and had not this genus already been adopted by several authors, I should not, I think, myself have formed it; though, no doubt, it harmonises well with some of the other genera of the family, which are perhaps all too intimately related. Creusia differs from Pyrgoma only in the shell being separated by sutures, into four compartments, with well developed radii: in other respects, such as in general habit, in the cup-formed imbedded basis (not permeated by pores), in the opercular valves, in the characters derived from the mouth and cirri, there are no generic differences. This affinity is more particularly evident when Creusia is compared with the first few species of Pyrgoma: indeed, for a short time, I was inclined to consider var. 10 of Creusia as identical with Pyrgoma conjugatum. With respect to the species of Creusia, I confess I have been much perplexed in determining whether there be only one, or half a dozen. The latter conclusion would almost certainly be arrived at if only a few specimens were examined; and it might naturally be thought that some of the species were extremely well marked; the difficulty of drawing any line between varieties and species, begins only when some hundreds of specimens, from various parts of the world, are disarticulated, cleaned, and carefully examined. Creusia, in this respect, offers a striking contrast with Pyrgoma, in which nearly all the species are strongly characterised. The shell differs very little in the several varieties or species of Creusia; and the most marked difference, namely, whether the walls are permeated by irregular pores or not, seems certainly quite variable. It is in the opercular valves, which in other genera offer by far the most reliable character, that we encounter the chief cause of perplexity; for the characters thus derived, though at first appearing very distinct, blend into each other, and are not accompanied by any well marked differences in the shell. Only a few of my specimens have any habitat; but the geographical range, as far as it does go, throws no light on the question which forms to regard as species and which as varieties. As is generally the case with cirripedes, the variations are local, so that the greater number of specimens imbedded in the same coral resemble each other. Under these circumstances I have thought it best, after repeated examinations of a very large suite of specimens, to describe separately each variety, without attaching any name to it; but I will first make a few general remarks on the structure of the shell. If I do not thus throw much light on the subject, I shall at least not burden it with error. I believe that the species will be definitely made out only by persons resident in the coral-bearing zones. I have given copious illustrations of the opercular valves; for, if my view be correct, this genus offers a curious and striking case of variation; if, on the other hand, I am wrong, the drawings, I hope, will aid others in coming to a more correct conclusion.
1. CREUSIA SPINULOSA. Pl. 13, fig. 6 a-6 h: Pl. 14, 6 i-6 u, 6 U.
CREUSIA SPINULOSA. Leach (!). Encyclop. Brit. Suppl., vol. 3, Pl. 57, 1824.
CREUSIA SPINULEUSE. De Blainville. Dict. Sc. Nat., Pl. 116, fig. 6.
CREUSIA GREGARIA. G. B. Sowerby (!). Genera of Recent and Fossil Shells, No. 18, Sept. 1823.
—— GRANDIS. Chenu. Illust. Conch. Tab. 1, fig. 2, sed non fig. 2 a and b.
Hab. — Philippine Archipelago, China, Singapore, Java, Red Sea, West Indies; imbedded in various corals; Mus. Brit., Cuming, Stutchbury, Dunker, &c.
General Appearance. — The shell is oval, generally flat, sometimes conical, with narrow and approximate ridges radiating from the orifice (fig. 6 a). The ridges, however, are sometimes distant from each other, and considerably prominent, projecting round the basal border. The orifice is either neatly diamond-shaped or oval. The four compartments are quite distinct; the radii are generally white, of considerable width, and with their summits not oblique. The colour is either white, or pale pinkish-purple; but in var. 11, bright pink. Even in the white specimens, when well preserved, the sheath is generally, but not always, either pale or dark purple. The largest specimen which I have seen, from the West Indies, was above half an inch in diameter; but from .3 to .4 of an inch is the more usual full size. I believe that the size, as well as the great variability of the present species, is partly determined by the rate of growth of the various zoophytes in which the specimens are imbedded, for the shell has to keep on nearly a level with the surface of the coral.
Structure of Shell and Basis. — The walls are internally ribbed; the ribs being usually prominent, sometimes to such a degree as to deserve to be called plates. The outer lamina is of variable thickness, and the prominence of the internal ribs appears in considerable part to depend on the extent to which the outer lamina has been thickened from within. In many specimens, instead of the interspaces between these internal ribs or longitudinal septa being solidly filled up, separate and successive laminæ have been deposited, by which the shell is rendered cancellated or porose; but the pores are very irregular; and sometimes they form two or three irregular rows one behind the other: this structure seems eminently variable. The edges of the radii are formed by crenated, and occasionally branched, septa. That part of the alæ, which is added during the diametric growth of the shell, is very thin. The lower edge of the sheath seems always to be free. The shelly layer, forming the basis, which is deeply cup-formed, is thin, more or less finely furrowed in radiating lines, and not permeated by pores.
The opercular valves will be best described under the following eleven varieties.
Var. (1), Pl. 13, fig. 6 a, 6 b, 6 c, 6 d. — Hab. Java, and probably several other districts. — I will first describe a typical sub-variety, by which I mean a sub-variety not presenting any extreme character. The scutum is of a sub-triangular shape, with the basi-tergal corner much rounded off (6 d), and generally but not always hollowed out in a rather remarkable manner. The adductor ridge is considerably prominent, and extends high up, parallel to the articular ridge, which latter is rounded and prominent, but to a variable degree. Near the rostral angle there is sometimes a small tooth, or only a trace of one, depending beneath the basal margin; this tooth we shall hereafter see much more developed. The Tergum is about two thirds of the width of the scutum. It is often slightly beaked, but this is more conspicuous in some of the following varieties. The spur is about half the width of the valve, and its basal end is truncated, and nearly parallel to the basal margin of the valve, but the truncated form passes insensibly into a rounded outline. The shell in this variety is generally thick and is not permeated by pores; the orifice is diamond-shaped. But in another sub-variety the walls of the shell are always, or nearly always, permeated by pores, and the tergum is very much narrower, with the spur sharper, so that at first I concluded that these two sub-varieties were specifically distinct: we shall, however, soon see in var. 2 and in var. 4, that no confidence whatever can be placed in the exact breadth of the tergum, or in the porosity of the walls; hence I have been driven to consider the two varieties just mentioned as merely sub-varieties.
Var. (2), fig. 6 e, 6 f, 6 g. — Hab. China; Red Sea. — The shell is almost invariably permeated by pores, sometimes arranged in two or three very irregular rows. In some specimens the scutum exactly resembles that in var. 1, but with the tooth near the rostral angle often rather larger: in other specimens the scutum is much more elongated transversely (fig. 6 e), with the adductor ridge more medial, and the basal margin not at all hollowed out at the basi-tergal corner of the valve. The tergum, here, is the remarkable feature, being sometimes excessively narrow, with a long sharp spur, which often, but not always, terminates in a needle-like point. In other specimens, from the same coral and certainly belonging to this same variety, the valve is not so narrow (6 g), and the spur not so pointed; consequently (as in several analogous cases in other cirripedes) it is impossible to draw any line of distinction between the specimens with the narrow and broad terga.
Var. (3), fig. 6 h [Creusia gregaria of G. B. Sowerby!] — Hab. Unknown. — The scutum presents here exactly the same considerable range of variation as in var. 2. The tergum is broad, as in var. 1, but the spur is rounded, and from not being placed so immediately close to the basi-scutal angle of the valve, gives to it a rather different aspect. The breadth of the spur varies; an extreme variety is given in fig. 6 h.
Var. (4), Pl. 14, fig. 6 i, 6 k, 6 l. — Hab. Philippine Archipelago; West Indies. — The scutum here presents the same sub-varieties as heretofore, excepting that I have not seen any so much elongated transversely. The shell is covered either with slight, closely approximate ribs, as in the foregoing varieties, or with more distant and more prominent ribs. In specimens taken out of the same branch of coral the walls were either porose or solid. Sometimes the sheath is bright purplish-pink. It is the tergum, again, which presents a remarkable range of difference; for the longitudinal depression or furrow which in the former varieties was quite open, here has its edges more or less folded inwards, and is sometimes quite closed. This same variation has been commonly observed in many species of Balanus, in which it appears to be dependent on the age of the individual; but this does not appear to be the case in the present genus. As a consequence of the greater or less folding in of the two sides of the furrow, the spur is rendered more or less narrow and pointed, and thus becomes removed to a greater or less distance from the basi-scutal angle of the valve. Further, as a consequence of this folding in, the internal surface of the valve along the line of the external furrow, is raised into a longitudinal ridge. The length of the spur varies considerably. In some very young individuals, the basal margin descends lower on the scutal than on the carinal side of the spur. In one set of specimens (fig. 6 l), a plate extended from the carinal margin to near the central longitudinal ridge just mentioned: a similar structure was described under Pyrgoma cancellatum.
Var. (5), Pl. 14, fig. 6 m. — Hab. Unknown. — We have seen in vars. 2 and 3 that the scutum varies considerably in shape: here it is unusually narrow, with the adductor ridge almost touching the articular ridge. There is no little tooth near the rostral angle, and the basi-scutal corner is not hollowed out. The tergum also varies; in some individuals it is truncated and like that figured of var. 1, but rather more rounded; in other specimens (from the same branch of coral) the basal margin so blends into the spur that the latter can hardly be discriminated (fig. 6 m); in other respects the outline resembles pretty closely that of one of the sub-vars. (fig. 6 f) of var. 2. The shell is not porose; it is thick, with strong internal ribs, and resembles that of var. 1; but it is of a pale purplish colour.
Var. (6), an. spec.? fig. 6 n-6 q. — Hab. Philippine Archipelago. — This is a very remarkable variety; we have, imbedded in the same coral, and with shells absolutely identical, specimens with the scutum having three distinct but graduated forms. Firstly, a scutum transversely elongated, in all external respects like some of the varieties mentioned under vars. 2 and 3, with no rostral tooth, and not hollowed out at the basi-tergal corner, but with the adductor ridge more prominent. Secondly, a scutum of the same general shape, but with the adductor ridge so much developed (fig. 6 n) as to descend slightly beneath the basal margin, and to be seen when the valve is viewed externally; there is a very slight tooth near the rostral angle (as in some former sub-varieties), and which can be here rather more clearly seen than hitherto, to be formed by the adductor ridge (closely united to the external surface of the valve) extending thus far, and being here produced a little downwards. Thirdly (6 p), we have the adductor ridge immensely developed, descending far below the basal margin of the ordinary valve; and the basal margin at the basi-tergal corner is angularly and deeply hollowed out. The appearance of the valve is widely different from that in the first sub-variety, yet it is impossible to separate the first and second sub-varieties, and it is almost equally certain that the third sub-variety is only an exaggeration of the second. The lower edge of the adductor ridge, in the third sub-variety, varies a little in outline; it is deeply sinuous, and is produced at the rostral angle into a point, of which we have heretofore seen only a feeble representation. It would appear as if the great development of the adductor plate had caused the exterior ordinary surface of the valve to shrink or be less developed. There is a striking resemblance in the structure here described with that in Pyrgoma cancellatum and conjugatum. The terga belonging to the above scuta, also, vary; the spur being sometimes square (6 o), and sometimes bluntly pointed: when the spur runs in the same exact line (6 q) with the scutal margin of the valve, a peculiar aspect is given to it, but this is by no means always the case. Both opercular valves are often partially coloured pinkish-purple. The shell is not porose; it is thin, with remarkably prominent internal plates; it is apparently always of small size, which I attribute to this variety inhabiting a hard thin plate-like coral. The sheath is bright pinkish-purple, of which we have had instances in some of the other varieties; and the shell itself is sometimes pinkish. Taking the scutum of the first sub-variety, together with the commonest accompanying variety of tergum, I find it quite impossible to assign to it a specific character; if, on the other hand, we consider the scutum of the third sub-variety by itself, nothing can appear more distinct; but I must repeat, there can be hardly a shadow of doubt that the three sub-varieties of scutum here described, graduate into each other, and are specifically identical.
Var. (7), fig. 6 r. — Hab. Probably Philippine Archipelago, associated with Balanus quadrivittatus. — There can be hardly any question of this being specifically identical with the last variety. It inhabits a different coral. All the specimens were of small size. The walls are not so thin, and the internal ribs not so prominent as in var. 6. The sheath is either white or dull purple; I can, in short, point out no difference in the shell from the typical var. 1. The scutum is not so much elongated transversely as in var. 6, and the basi-tergal corner is more cut off, — in which respect it resembles the common varieties. The adductor ridge is largely developed, so as to be just visible when the valve is viewed from the outside, in a degree between the first and second sub-varieties of var. 6: but the most singular character is the larger development of the tooth near the rostral angle, and this was the case in the same degree in all the specimens which I examined. The tergum resembles that rather unusual sub-variety of var. 6 (fig. 6 q), which has the scutal margin and the one side of the spur forming a straight line. It appears to me that it would be absurd to consider these slight differences, in parts unquestionably subject to much variation, as specific, when we are almost forced to admit that the much greater differences in the three sub-varieties of var. 6, are not of specific value.
Var. (8). — Hab. Unknown, Mus. Cuming. — I have seen only a single specimen of this, and refer to it on account of var. 11. The shell is rather steeply conical, with distant and prominent ribs; the radii are narrow; the walls are not permeated by pores; the colour is pale purple. Altogether its external appearance is very different from that of the foregoing varieties; but the scuta are identical with those of var. 1, excepting that the rostral tooth is rather larger, being nearly as large as in the last, var. 7. The tergum precisely resembles that in some specimens of var. 2. Hence this variety differs from the first two varieties only in the shade of colour, the external shape, and the greater prominence of the external radiating ribs of its shell. All these characters are variable in the several foregoing varieties, and they have been found, as yet, insufficient to discriminate species in any genus of sessile Cirripedes.
Varieties With the Scuta and Terga Calcified Together.
Var. (9), Pl. 14, fig. 6 s [C. spinulosa, of Leach (!)] — Hab. Unknown. — The shell is undistinguishable by a single character from many specimens of the first, third, and fourth varieties; it is not permeated by pores. The scutum and tergum, with the exception of the one striking difference of their being calcified together without any trace of a suture, are identical with those of var. 3, as may be seen by comparing the figures 6 h and 6 s. Hence to separate this form specifically from var. 3, we should have to rely solely on the calcification or union of the scuta and terga; but we have seen this is a point which is variable in Elminius Kingii, Pyrgoma milleporæ, and in some species of Balanus. The serial affinities, moreover, in Pyrgoma, clearly show that this is a character of no great importance. I must add that in several specimens of several of the varieties, the scuta and terga were so closely joined, that until careful examination, I was unable to detect the suture separating them; such being the case it must be quite unimportant for any functional purpose, whether or not the valves are calcified together. I feel, consequently, hardly any doubt that I have acted right in treating the present form as a mere variety.
Var. (10) an. spec.? fig. 6 t. — Hab. Unknown. — This variety bears nearly the same relation to var. 6, as the last variety did to var. 3. The shell is rather stronger than in var. 6, with the internal ribs not so prominent; and except in being tinted pale dull purple, it differs in no respect from the shell of var. 1. If we imagine the scutum and tergum in the third sub-variety of var. 6 (6 p, 6 q), in which the adductor ridge descends far beneath the true basal margin of the valve, to be calcified together, without any suture, we shall produce almost the identical valves of the present variety. The scutum, however, here is not quite so much elongated transversely, and the occludent margin is spinose and is furnished with large teeth; these two characters give the valve a somewhat different aspect, and hence I am more doubtful than in the foregoing case, whether this form may not be specifically distinct. I must, however, state that in Tetraclita porosa, I ascertained that the teeth on the occludent margin of the scuta were even more variable than here is supposed to be the case; and as for the shape of the valve we have seen what wonderful variation there is in var. 6. The tergum in this variety is about intermediate between the two common forms, in the sub-varieties of var. 6. As for the calcification of the two valves together, we have seen, under the last variety, how little important a character it is.
Var. (11) fig. 6 U, 6 u [Creusia grandis, of Chenu, Tab. 1, fig. 2, but not fig. 2 a and b]. — Hab. Singapore, associated with Pyrgoma monticulariæ. — This variety is very closely related to the last. The shell, however, has a very peculiar aspect, which made me for some time think it must be specifically distinct. It is of a much brighter pink than in any of the foregoing varieties; the surface is marked with very prominent, distant ribs, and the radii are narrow, in which latter points, together with the tint (though here brighter), this variety cannot be distinguished from var. 7. The shell, however, is permeated by several rows of pores, in which respect it resembles the shell in var. 2, and some specimens of vars. 3 and 4. In the opercular valves there is a close general resemblance with those of the last var. 10; the tooth, however, near the rostral angle, is not so prominent; and in the tergum, the spur is more truncated, shorter and broader than in var. 10, and closely resembles that in var. 1 and 2. But I cannot consider any of the points here specified of much weight.
The foregoing descriptions show how singularly the affinities of the several varieties interlock in the most complicated manner. Hereafter some one may, perhaps, succeed in grouping several of these forms as species; but I am sure he ought not to attempt it without possessing a very large suite of specimens, or without the great advantage of comparing some two or three of the forms, fresh in their native site.
Species dubiæ. — Under Pyrgoma, I have stated that though Chenu, in his ‘Illust. Conch.,’ has given beautiful external figures of the shells, imbedded in the coral, yet from the want of details on the opercular valves and on the structure of the shell, I cannot recognise his species. So it, likewise, is with Creusia. Chenu gives the following new species; C. radiata, multistriata, decorata, and striata. The C. madreporarum, I suspect to be the Pyrgoma milleporæ of this work, as there stated. The C. grandis no doubt is the P. grande of this work, the Nobia grandis of Sowerby. The name Creusia Childreni is given by Dr. Gray, without description or figure, in the ‘Annals of Philosophy,’ vol. 10, new series, 1825.