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;[115] Brit. Mus. and Stutchbury.
[115] 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.