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.