Cirri: first pair short, with the rami equal in length. Second pair, with the anterior ramus having broader segments than those of the posterior ramus, and with all the segments, except the few uppermost, thickly covered with spines; the posterior ramus has rather less than half the segments thickly covered. Third pair, very slightly shorter than the sixth pair; anterior ramus with the lower segments, less than half of the whole in number, thickly covered with spines; posterior ramus with only the lowest segments, about one fifth of the entire number, thickly covered; the other segments of these two rami, and the upper segments of the posterior ramus of the second pair, closely resemble in the regular arrangement of their spines in pairs, the three posterior pairs of cirri. The pedicel of the third pair supports numerous, irregularly scattered bristles. The segments of the sixth pair bear four or five pairs of main spines, with a few intermediate spines.
Caudal Appendages.—Multiarticulate, narrow, tapering, situated on each side of the anus: each segment has two little tufts of spines on each side of its upper edge. These appendages are about one third longer than the pedicel of the sixth pair of cirri: in a specimen, in which the rami of the sixth pair had twenty-three segments, the caudal appendages had nineteen segments.
Penis short, hairy, finely-ringed, with no projecting point at its dorsal basis. Branchiæ moderately large, nearly circular, not much plicated.
Fossil Specimens.—With respect to the Sicilian specimens sent me by Dr. Philippi and Sir C. Lyell, I can see no difference whatever from the recent specimens, excepting in their greater size and thickness; it must, however, be borne in mind that I have seen only half-a-dozen recent shells. The one fossil scutum which I have seen is rather broader than is usual with the recent, but I have seen one nearly as broad. This species seems to have been extremely common when the beds at Messina were deposited, and probably it attained a larger size than it does at present. The compartments are always found separated, which is easily understood, by the facility with which, as above stated, recent specimens fall to pieces.
2. [PACHYLASMA] AURANTIACUM. Pl. [20], fig. [1 a]-[1 d].
Shell tinged with orange; viewed externally seems formed of only four compartments, owing to the carino-lateral and lateral compartments on each side being separated only by an obscure fissure.
Hab.—New South Wales; apparently from deep water, attached to sandstone.[135]
[135] I am indebted to Mr. Bowerbank for this unique and interesting species, which I have deposited in the British Museum.