2. [CHTHAMALUS] ANTENNATUS. Pl. [18], fig. [2].
Shell conical, generally smooth: when not deeply corroded of a pale dirty flesh-colour: sutures always distinct: radii, when present, with their sutural edges quite smooth.
Hab.—New South Wales, (Moreton Bay, 27° S.; Sydney; Twofold Bay), Van Diemen’s Land (Hobart Town). Attached to littoral rocks and shells; Mus. Brit., Cuming, Darwin.
General Appearance and Structure of Shell.—Shell conical, rather smooth; when not much corroded, of a pale dirty flesh-colour; often covered by membrane; sometimes deeply corroded, extremely rugged, and then of a brown colour; in this condition not much punctured, as generally is the case with [C. stellatus]. Sutures almost always quite distinct; rarely the shell becomes cylindrical with the sutures obliterated. Orifice moderately elongated, sub-hexagonal. Radii rather narrow, but not so narrow as in [C. stellatus], smooth, with their upper margins very oblique: when disarticulated their edges are quite smooth. The edges of the alæ are sometimes crenated, and sometimes not so, being only marked by lines of growth; they are often rather thick. The parietes are usually rather thick, with their internal surfaces smooth, and not mamillated, as is so often the case with [C. stellatus]. The largest specimens which I have seen, were .6 of an inch in basal diameter.
Opercular Valves.—These are hardly distinguishable from those of [C. stellatus]. The only very slight difference which I can point out is, that the crests for the tergal depressores are less spread out, and depend rather more beneath the basal margin of the valve; and lastly, that the surface of the tergum, just above these crests, is rather prominent.
Branchiæ: these are oblong; taper but little, and have a broad rounded end: they are scarcely plicated.
Mouth.—The crest of labrum is hairy: the palpi are square, and have no bristles along their basal exterior margins, but long ones at their truncated ends. The mandibles have three or four main teeth apparently single: the inferior coarsely pectinated portion is short. The maxillæ are deeply notched.
Cirri.—The first and second pairs, and portions of the third, are darker coloured than the three posterior pairs. The rami in both of the first two pairs are slightly unequal in length. The third cirrus is much longer than the second: in a specimen in which there were six segments in the shorter ramus of the first and second pairs, there were twenty segments in the posterior and shorter ramus of the third cirrus; and in this same individual there were no less than forty-nine segments in the anterior ramus. In another specimen (Pl. [29], fig. [2]) the number of segments in the two rami of the third cirrus, was 20 and 41; in another, the numbers were 18 and 53; in several other specimens the numbers varied in about these proportions; but in one single specimen the numbers were equal. Not only did the number of segments thus vary in the two rami, but likewise the arrangement of the spines on the segments in the anterior and longer ramus; in some specimens the spines on all the segments were arranged in a single circle, and then the organ had a specially antenniformed appearance: in other specimens, some of the lower segments (in one case thirteen in number) had the spines placed in regular pairs precisely as on the posterior ramus, and as on the three posterior pairs of cirri. Under the genus I have pointed out the resemblance between this structure and that occurring in certain Macrourous Crustaceans. The pedicel of the third cirrus had its spines more crowded and irregular than on the three posterior pairs of cirri. The segments in the latter vary in bearing either three or four pairs of main spines. The whole dorsal surfaces of the lower segments of the several posterior cirri are serrated in an upward direction by short spines, but to a variable degree.