Structure of Shell.—The longitudinal ribs on each compartment (i. e. the terminal transverse loops), are convex and prominent, as in C. diadema, but they are crossed by more prominent ridges of growth than even in the roughest varieties of that species, so that the surface of the shell is more rugged. In the three previous species, the surface of the wall entirely round the cavities occupied by the whale’s skin, is striated only by longitudinal very fine lines; but here, the outer portion, or that (fig. 6) formed by each transverse loop, is crossed by transverse ridges of growth, like, but less prominent than, those on the external surface of the shell. The minute teeth, along the lines of junction between the transverse loops, are here less regular, and can hardly be said to exist; for the two edges are locked together by what may be more strictly called minute zig-zag ridges than teeth. The exact number of the circumferential plications in the walls is variable in the same manner as in the three foregoing species. The sutural edges of the radii are about as thick as, or rather thicker than, in C. diadema. As in this latter species, and in C. reginæ, each ala rests, not on the internal surface (as in C. balænaris, and in all other Balanidæ) of the radius, but on a special plate; but in C. barbara, instead of a deep chamber, running up to the apex of the compartment, being left between the radius and ala, this part is filled up almost entirely by solid shell. Although the extent to which this chamber is filled up varies a little, and although its depth varies a little in C. diadema, yet there is a marked difference between the specimens of this latter species, in which the chamber is most filled up, and those of C. barbara, in which it is least filled up. The alæ are thick, as in C. diadema, and their sutural edges have a central ridge, sending off on both sides sinuous ridges. The basal margins of the alæ are not short compared with their upper margins, and therefore the whole ala is not wedge-formed; and in this rather important respect C. barbara resembles C. balænaris (Pl. 16, fig. 3), and differs from C. diadema (fig. 2). The lower edge of the sheath does not seem to have projected freely,—in this respect, also, resembling C. balænaris. From the basal margin of the alæ not being narrow, and from the inner ends of the folded walls being, as it would appear, also broad, I have little doubt that the cavity in which the animal’s body was lodged, resembled in shape that in C. balænaris, the membranous basis being larger than the orifice of the shell.

Opercular valves unknown.

Summary.—This species is more nearly related to C. diadema than to the others; but in some points, just specified, it resembles C. balænaris. The characters by which it differs from all the species are, firstly, the more prominent transverse ridges on the external surface of the shell, and more especially on the surfaces bounding the outer sides of the cavities occupied by the epidermis of the whale. Secondly, the character of the teeth, or rather ridges, along the lines of junction between the transverse loops. And, thirdly, the spaces between the radii and the plates on which the alæ rest, being solidly filled up.