ACASTA GLANS. _Lamarck._ Animaux sans Vertèbres, 1818.
_Parietes internally quite smooth, with the lateral margins of each
compartment inwardly prominent: basis with the edge rarely crenated,
but furnished with six inwardly prominent teeth: scutum strongly
striated longitudinally._
_Var._ (_a_) _with the edge of the basal cup finely crenated_.
_Hab._--New South Wales, Southern Australia; Mus. Brit., Stutchbury,
&c.
This fine species seems to be extremely common, imbedded in an open
porose sponge on the eastern and southern shores of Australia. It
is very distinct from the other species, with the exception of the
following _A. lævigata_, which, with some hesitation, I have allowed to
remain specifically separated.
_General Appearance._--Excepting in its larger size, this species
differs in external appearance but little from _A. spongites_; its
colour is pale dirty reddish. The surface is generally studded
with small calcareous points. The parietes of the carino-lateral
compartments are about one fourth of the width of the parietes of the
lateral compartments, and therefore proportionally of the same width
as in _A. cyathus_. The largest specimen which I have seen, was .55
of an inch in basal diameter.
_Scuta._--These are slightly narrower, thicker, and more convex than
in _A. spongites_: they are strongly striated in longitudinal lines.
The articular ridge is very feebly developed. _Terga_: in full-grown
specimens, the spur is half the width of the whole valve, and is
truncated; its basal edge being parallel to the basal margin of the
valve. The articular ridge and crests for the depressor muscles are
very feebly developed.
_Internal structure of the parietes._--The inner surface of the
parietes is quite smooth, without even a trace of ribs or teeth. But
the most important character is that the internal lateral margins on
both sides of each compartment, from the sheath to the basis, project
inwards and form a rim; so that when the shell is viewed from within
(Pl. 9, fig. 5 _b_, representing the lateral and carino-lateral
compartments, and part of the carina), the six sutures are seen to be
strengthened by six double columns.
_Basis._--This is moderately cup-formed. The edge, in order to
meet the basal points of the inwardly projecting lateral margins
of the six compartments, has six knob-like teeth. These are placed
at unequal distances, for two on each side stand near each other,
owing to the narrowness of the carino-lateral compartments. The
degree of their development varies extremely; when most developed,
as in the specimen figured (Pl. 9, fig. 5 _a_), each tooth is bifid
and a little hollowed out, so as to receive the points of the two
inwardly projecting margins which form each suture. Ridges, more or
less prominent, running from each of the six marginal teeth, extend
towards the centre of the cup. These six teeth cannot be seen from
the outside. The edge of the cup is rarely crenated; but I have seen
two instances in which this was the case.
_Cirri._--In the first pair, the rami are not quite so unequal as
in _A. spongites_; the longer ramus being about twice as long as
the shorter. In the third pair, there are some very minute, thick,
upwardly-pointing spines, which I did not notice in _A. spongites_.
In the fourth pair, the spines are a little more crowded, with longer
dorsal tufts, than in the sixth pair; and they are mingled with some
very minute, thick, upwardly pointing spines. In young individuals,
there are only three pairs of main spines on the segments of the
sixth pair, instead of four pairs.