Var. (a), epidermis coloured dull orange.—Red Sea.
Hab.—Red Sea, Philippine Archipelago; Mus. Brit., Cuming, &c.
This species, of which I have examined many specimens from the above two and other unknown localities, agrees in all essential points of structure with [A. glans], and consequently I for some time classed them together; but the characters, though usually of small value, by which this form differs from [A. glans] being apparently constant, I have with some doubt allowed it to remain specifically distinct. These characters are, firstly, the much smaller size of the whole shell in [A. lævigata]; secondly, the edge of its basal cup being always crenated, which seems to be a rare accident in [A. glans]; thirdly, though of secondary importance, the scutum being here less plainly striated; and, lastly, the spur of the tergum being of less breadth, and of a more rounded outline; on the other hand, it must be confessed, that when small specimens of [A. glans] are taken, there is hardly any difference in the spurs of the terga.
General Appearance and Structure of Shell.—The surface of the shell is often very smooth, but is sometimes studded with some small sharp calcareous points. The colour is white, or pale reddish-brown; but in the specimens from the Red Sea, the tint is more orange, with the upper part of the shell white. The orifice of the shell is unusually small. The largest specimen which I have seen was only .25 of an inch in basal diameter, and therefore less than half the size of [A. glans]. The internal surfaces of the parietes are smooth, with the two lateral margins inflected, as in [A. glans]. The edge of the basal cup has six knob-like teeth, like those in [A. glans], but smaller; and, in addition, it is finely crenated.
Scuta: these differ only in being less plainly striated in longitudinal lines; indeed, some specimens show hardly a trace of this structure. Terga; these valves, in some varieties (Pl. [9], fig. [6 b]) can hardly be distinguished from those of equal size from young individuals of [A. glans]; other varieties have the spur (Pl. [9], fig. [6 a]) not truncated, but broadly pointed, and therefore of considerably different shape.
Neither in the mouth, nor cirri could I detect any difference with [A. glans].
7. [ACASTA] FENESTRATA. Pl. [9], fig. [7 a]-[7 c].
Shell reddish, with six large, membrane-covered apertures between the sutures, above the basal cup: carino-lateral parietes half as broad as lateral parietes; internally, parietes and edge of basis smooth; tergum with the articular ridge short and prominent; spur pointed.
Hab.—Philippine Archipelago, Mus. Cuming.