PYRGOMA (DARACIA) MONTICULARIÆ. J. E. Gray (!). Zoological Miscellany, p. 6, 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.