General Appearance and Structure of Shell.—The sutures separating the four compartments are generally, excepting at an early age, quite obliterated, both internally and externally, the shell in this case consisting of a single piece, with its summit and opercular valves always much worn. Occasionally the sutures are preserved, and then the four compartments are seen to be of nearly equal sizes. The orifice is always broadly oval, with the carinal end the broadest; and it often approaches closely to circular. The upper part of the shell is frequently steeply conical, with the lower part spreading and folded; sometimes deeply folded. Very young shells are apt to be remarkably smooth. The radii appear never to be developed: the inside of the shell is smooth. The colour is either blackish-green (this being the tint of the corium lining the sack), with the upper part gray from disintegration; or the lower part is brown, from the investing membrane, the shell itself being pale coloured. This species seems particularly liable to grow crowded together; often covering rocks and shells with a honey-combed layer. The basal diameter of some of the largest specimens was .3 of a inch. In Mr. Cuming’s collection, however, there is an Australian specimen .55 in diameter and .3 in height, which is, moreover, remarkable from the projecting, extremely rugged, overlapping, dark-coloured layers of growth, which surround the lower part of the shell: we have seen that Spengler’s specimens, said to have come from Otaheite, are even broader and considerably higher.

Scuta: from the disintegration which the valves have undergone, the scuta and terga are externally seen to be locked together by a deeply sinuous articulation. The Scuta have a wide articular furrow and a very prominent articular ridge; but the exact outline of these parts varies greatly, very much as in the genus Chthamalus.

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.

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.

Structure of Shell.—The rostrum (fig. 4 c) is remarkable from its small size, and from the plainness of its sutures, in comparison with those separating the other compartments, and this is exactly the reverse of what I should have expected in a compartment tending to become rudimentary. Not only is the rostrum small, but the alæ project to an unusually small degree, and gradually slope away into the lower part of the parietes. These peculiarities are even more strongly marked in very young shells: thus in one of the size of a pin’s head, the rostrum consisted of a minute parallelogram, without, as far as I could see, any alæ, and was only one fourth of the size of the carina,—this latter compartment being only half as wide as the lateral compartments. The carina, at this early period, had quite distinct alæ.

The tubular prolongations from the four external holes are of course very conspicuous on the under side of the shell (4 b); and their structure is there plainly seen. In extremely young shells the holes are not present; but very soon, at four points of the circumference, namely, two in the carina and one in each lateral compartment, the basal edge becomes indented, and during growth more and more deeply indented; at last the horns or points of the bays, thus formed in the circumference of the shell, grow inwards and meet, the four indentations being thus converted into four rings or holes; as the shell is added to, at its circumference, these come to stand further and further from the exterior margin; and as the shell at the same time rises above the surface of attachment, the holes are added to at their basal edges, and are thus converted into shelly tubes, generally freely open at the bottom as well as at the top. Sometimes these tubes are closed at the bottom, and this is usually caused by their sides having been added to in a spiral direction. A somewhat sinuous double ridge or fissure, leading from the tubes or holes to the exterior border, can always be perceived on the under side of the shell. Occasionally, though rarely, in very old shells, a second series of holes is formed outside the first four holes, and often a tendency to this may be perceived in the just-mentioned fissures expanding a little at their outer ends, thus forming four new circumferential indentations. The purpose of this peculiar structure, apparently, is to give support to the much depressed and thin shell. In Chthamalus stellatus and scabrosus, we have seen a slight indication of a similar structure, in the formation on the under side of the shell, but confined to the lines of sutures, of obscurely tubular pillars: we have also something analogous in the singular midribs, in Platylepas, causing the membranous basis to be convex. I need only further add, that the parietes, in Chamæsipho, are rather thin, and are composed of translucent shell, punctured for the entrance of tubuli, with the punctures often arranged in straight lines.

Opercular Valves (fig. 4 d).—These are attached by a narrow opercular membrane to the sheath, but little beneath the summit of the shell. The Scuta are considerably arched or convex: the articular ridge is very prominent, and there is a thick strong adductor ridge. The Terga have a short, rather broad, rounded spur, placed very nearly in the middle of the valve: the crests for the tergal depressores are moderately developed, and are simple.

Mouth.—The crest of the labrum is hairy, and is much hollowed out. The palpi are small and narrow, with long bristles at their apices. The mandibles have four or five teeth, with the lower part pectinated. The maxillæ are notched. Of the Cirri, the second pair is short, and all the segments are thickly clothed with bristles. The third pair in the arrangement of the bristles resembles the four posterior pairs. The segments in these pairs are elongated, and support four pairs of main spines.

I was not able to observe any Branchiæ.