I am almost ashamed to admit this species, so small are its differences compared with [A. spongites]; yet I think that it probably is a distinct form.
In general appearance and character this species comes very near to [A. spongites]. As in the latter, the parietes of the carino-lateral compartments are narrow.[104] The orifice seems always to be smaller. Internally, the parietes are generally much more strongly ribbed, and the edges of the basal cup more plainly crenated. The sheath is generally coloured of a brighter pink, sometimes tinged with orange. The average largest specimens (from .3 to .5 of an inch in basal diameter) are a little larger than the largest European specimens: I have seen one specimen from Moreton Bay .4 in basal diameter, and from the basal cup being very deep, actually .75 in height. The scutum has the articular ridge not so prominent and not so abruptly cut off at the lower end, as in [A. spongites]: on the other hand, the adductor ridge seems rather more prominent; but these differences are trifling. In the tergum the breadth of the spur (fig. [2 c], [2 d]) varies in specimens taken out of the same branch of sponge; some can hardly be distinguished from the same valves in [A. spongites], but generally the spur is broader and squarer.
[104] In Mr. Cuming’s collection there is a specimen, from Sydney, which I fully believe to be the present species, but cannot be positively sure, as the opercular valves have been lost, which is very remarkable from the walls of the carino-lateral compartments being reduced to the thickness of a mere thread, not one twentieth of the width of the lateral compartments; we here see the structure of [Acasta sporillus] prefigured.
This species presents some remarkable varieties: in one specimen, from Sydney, the parietes were externally ribbed longitudinally (fig. [2 b]), the ribs being roughened with minute points, giving to the shell an elegant appearance; and this without doubt is the A. sulcata of Lamarck, procured by Peron at Shark’s Bay, lat. 25° S., on the opposite of the Australian continent: some specimens from Port Lincoln, in South Australia, were not ribbed, only smoothly striated in longitudinal lines; although both sets of specimens had almost smooth scuta, and were thus different from common specimens, yet there could be no doubt, from their similarity in all other points, that they did not differ specifically from them, though the latter had their scuta striated, but not their parietes. Hence we see that there is no relation between the striæ on the parietes and on the scuta. The Port Lincoln specimens, and some others, were remarkable from the radii not extending down to the basal cup, a minute cleft, covered only by membrane, being thus left along the sutures, low down between the parietes (fig. [2 a]); we shall see this singular structure strongly developed in A. perforata. Owing apparently to these clefts, the edge of the basal cup, exhibited traces of six knob-like teeth, like those characteristic of [A. glans].
Cirri.—The cirri resemble those of [A. spongites], with the exception that the segments on the posterior pairs bear only three main pairs of spines. With respect to the anterior ramus of the fourth pair of cirri, the following very singular facts were observed:—in a specimen from New South Wales (var. with the tergum having a narrow spur), on two or three of the lower, but not on the lowest, segments, the front margin was produced or developed into two or three minute, thick teeth, slightly curved like hooks downwards: in other specimens from New South Wales (var. with the tergum having a broad spur, and inhabiting the same branch of sponge with the last-mentioned variety), there was no trace of these teeth. But again, in two other specimens with the tergum having a broad spur (collected by different persons, near Sydney), and in another from South Australia, this structure was carried to an extreme, for in these (as represented, Pl. [29], fig. [2]) there were beautifully formed teeth on the fourteen lower segments (the twelve upper being without them), and likewise on the upper segment of the pedicel. These teeth are graduated in size on each segment; they are admirably adapted for securing any prey; and, in fact, they convert each segment into a mandible-like organ. On the segments, on which these teeth are well developed, some of the regular pairs of spines are aborted.
Diagnosis.—Finally, this species, if it be, as I believe, distinct, differs from [A. spongites] only in the internal surface of the parietes being more strongly ribbed and brighter coloured,—in the edge of the basal cup being more plainly crenated,—in the articular ridge of the scutum being of a different shape,—in the spur of the tergum being often broader,—in the segments of the posterior cirri having only three pairs of main spines,—and, lastly, in the occasional presence of the hook-like teeth on the anterior ramus of the fourth cirrus.
3. [ACASTA] CYATHUS. Pl. [9], fig. [3 a]-[3 c].
Carino-lateral parietes about one fourth of width of lateral parietes: radii wider than the parietes: basis nearly flat, small: tergum with the spur truncated, half as wide as valve.