The Terga are very narrow, with the under surface channelled: the attachment of the depressor muscle offers the only peculiarity,—the muscle being attached to four or five little pits, placed transversely to the longer axis of the valve; the septa between these little pits evidently answering to the crests as usually developed.
Mouth.—The crest of the labrum is hairy, and slightly bullate. The palpi are rather small, with long spines at their ends. The mandibles have four (sometimes five) teeth, with the inferior portion pectinated. The maxillæ are notched.
Cirri.—The first pair presents no remarkable character. The second and third pairs are subject to extreme variation, as in [Chthamalus antennatus] and [Tetraclita porosa]. In all the specimens, the anterior ramus of the second cirrus is short, with all the segments thickly covered with bristles; it is the posterior ramus which varies so much in relative length and in the arrangement of the bristles; but in no case are all the segments clothed with bristles as on the anterior ramus, and as is normally the case with all sessile cirripedes. In some specimens from New Zealand, the anterior ramus having only five segments, the posterior ramus was twice as long, having sixteen segments, with the bristles arranged in circles, but standing rather thicker together on the basal segments. In a Tasmanian specimen, the posterior ramus was only a little longer than the anterior ramus, and the spines were arranged in regular pairs (as on the three posterior pairs of cirri) on all the segments, excepting the few basal ones, on which they were more crowded: a nearly similar arrangement occurred in some other specimens from unknown localities, excepting that the rami were of nearly equal length. In the third cirrus, in all the specimens, the anterior ramus has three or four of its basal segments much broader than the upper segments, and thickly clothed with spines (as is the case with one species of [Chthamalus], viz. [C. intertextus]) all the other segments having regular pairs of spines. The posterior ramus of the third cirrus varies in being either much longer than, or only equal in length to, the anterior ramus; in the former case (in the New Zealand specimen) the spines were arranged in circles, giving an antenniformed structure to the ramus; and in the latter case they were arranged in regular pairs. In Mr. Cuming’s great Australian specimen, there was a further peculiarity, in the presence on the posterior cirri of a tuft of intermediate spines between the main pairs; and, in there being on those segments, which are thickly covered with spines, certain very large spines, doubly pectinated, with the pectinations elbowed, closely like the spines met with on the cirri in certain species of Pollicipes. Finally, the segments in the three posterior pairs of cirri support five or six pairs of main spines; the dorsal surfaces of the segment are rough and hairy.
Branchiæ, rudimentary; consisting of a small, simple, tongue-formed fold, projecting about 1/100th of an inch. Ova, 16/2000ths of an inch in length.
2. [CHAMÆSIPHO] SCUTELLIFORMIS. Pl. [19], fig. [4 a]-[4 d].
Rostrum very small, elongated, triangular: lateral compartments, each with an aperture, and carina with two similar apertures, all four leading into shelly tubular columns.
Hab.—Attached to Pollicipes mitella, probably from the seas of China; Mus. Brit.
General Appearance.—This very singular shell would not, without some examination, be thought to be a cirripede. From the symmetrical position of the four apertures, with the diamond-shaped orifice in the middle, with the sutures on each side of the rostrum, and from its depressed and circular form, this shell bears some resemblance to the perforated species of Scutella. Shell much depressed, generally nearly circular, with the basal margin highly sinuous and even sometimes almost branched. Surface slightly irregular, marked by fine lines of growth, and covered by brown membrane. Of the four compartments, the Rostrum is very narrow, triangular, and comes up to the orifice almost in a point: it is rather depressed, that is, it lies rather below the level of the other compartments: the straight sutures separating it from the lateral compartments are distinct in the upper part, though always obliterated in old shells in the lower part: these sutures are generally far plainer than those separating the lateral compartment from the carina, which in most cases are obliterated and calcified together, excepting close to the orifice. The alæ of the rostrum are not externally visible, and there are no radii to any of the compartments. The Carina is twice as broad as the rostrum, and is furnished with alæ of the usual shape, which are, to a certain extent, externally visible. The Lateral Compartments are broad, being broader than the carina; they are both penetrated, down to the surface of attachment, by a hole or rather tube,—the two holes standing opposite the rostral end of the operculum: the carina is penetrated by two rather smaller but similar holes. It is these four holes which give to this cirripede its very singular aspect: they are rather smaller than the orifice of the shell; they are oval, with their longer axes placed in the direction of the ray of the circular shell: their manner of formation will be immediately explained. The orifice is neatly diamond-shaped and broad: it is rather small compared with the whole shell, and is closed by the operculum, which is seated near the summit of the shell. Basal diameter of largest specimen 2/10 of an inch; few, however, attained this size, and perfect larvæ were included in much smaller specimens.