ACASTA LÆVIGATA. _J. E. Gray_ (!). Annals of Philosophy, (new
series), vol. 10, Aug. 1825.
_Parietes internally quite smooth, with the lateral margins of each
compartment inwardly prominent: basis with the edge strongly crenated,
and furnished with six inwardly prominent teeth: scutum feebly striated
longitudinally, or smooth._
_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_.