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.
6. [ACASTA] LÆVIGATA. Pl. [9] fig. [6 a], [6 b].
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.