Having described, under the genus, in so much detail the structure of the whole shell, it will be sufficient here to point out the characters by which this species differs from the others.
General Appearance.—The shell is generally much depressed, though sometimes, in large specimens, from the turning in of the basal edges, a considerable degree of convexity is acquired. The radii are moderately wide, and give a star-like appearance to the shell. The surface of the whole upper part of the shell is smooth; the broad and much flattened ribs (i. e. the terminal, transverse loops of the folded walls) of which each compartment is formed, generally divide at a short distance from the apex. The close sutures separating these ribs are straight, and not finely serrated, by which character alone this species can at once be discriminated from the others. The surface, when closely examined, is found to be finely striated longitudinally, and is transversely crossed, chiefly in the lower part, by minutely beaded growth-ridges. The orifice is rounded-hexagonal, and is small compared with the whole shell; it is also smaller than the basal edge of the internal cavity of the shell; consequently the whole basal edge cannot be seen through the orifice from one point of view,—the operculum of course having been removed. An unusually large specimen was 2.75 of an inch in diameter; this individual was one of the more convex varieties, and yet its entire thickness, from top to base, was only .9 of an inch.
Operculum.—Having described, under the genus, the general structure of the opercular valve, I will here only add a few details on their shape. They are small compared with the whole extent of the opercular membrane. The scuta (Pl. [15], fig. [2 b], scutum to the left) stand near each other at the rostral end of the aperture leading into the sack, with their two rostral ends united by the yellowish-brown, longitudinally plicated, horny membrane, described under the genus. The terga (fig. [2 b]) stand a little apart from the scuta, on the sides of nearly the middle of the sack-aperture. The lips are protuberant and moderately developed all round. The scuta are elongated, and a little curved; including the upper imbedded portion, they are almost sub-triangular: but the under growing surface is much elongated, nearly flat, with the two ends of nearly the same width, truncated and rounded. The Tergum, in rather small specimens is, if the upper imbedded portion be included, sub-triangular, with the growing surface oval, and between one third and one half of the length of the scutum; but in large specimens, the tergum becomes style-formed, lying parallel to the tergal margin of the scutum, with the growing surface proportionally much smaller, and not above one fifth of the length of the growing surface of the scutum. Hence we see some tendency in the tergum to become rudimentary, as it is in the next species. The brown, horny, plicated substance in which the terga are imbedded, extends considerably beyond the valves themselves.
Structure of the Shell and Radii.—I have already so fully discussed this subject, that I will here only enumerate the points in which this species differs from [C. diadema] and [reginæ]:—Firstly, the more symmetrically folded walls (Pl. [16], fig. [5]), new folds arising on both sides of all six sutures. Secondly, the inner ends of the folded walls, which surround the internal cavity, being almost square, but with their angles rounded; their inner ends descend some little way beneath the basal edge of the sheath, as low, or lower, than the circumference of the shell. Thirdly, the external lines of junction between the transverse terminal loops being smooth or not serrated. Fourthly, these loops being flatter towards the outside, and being internally filled by septa (Pl. [15], fig. [2 a]); tubes being thus formed larger than the proper parietal pores. Sixthly, the sutural edges of the compound radii (Pl. [16], fig. [3]) being very much broader in the inner and upper part of each compartment than in the outer and lower part; for in the inner and upper part they stretch from the outside of the shell to the sheath, so that the alæ rest on them, and hence no large open cavity is left between the radii and alæ; in the inner and upper part, also, the radii extend down nearly to the basal edges of the folded walls. The septa, of which the radii are formed, stand further apart than in [C. diadema] and [reginæ]. Seventhly, the alæ are only 1/5th or 1/6th of the thickness of the radii, whereas in the two other species they are very much thicker, being nearly as thick as the radii: the alæ are also here (fig. [3], a′) squarer than in those two species, that is, their basal margins are not so short compared with their upper margins; their edges present, also, a slightly different structure. Eighthly, the basal edge of the sheath does not here project freely, and does not descend quite so close to the basal membrane.
Mouth.—The palpi are furnished with a row, extending along the whole basal exterior margin, of very long upwardly pointing spines; on their upper edges there is a brush of small spines. The edge of the labrum is clothed with extremely fine spines, and is furnished with a few minute teeth. The mandibles have four teeth, and the lower end is broadly rounded and coarsely spinose: the second and third teeth are double at their points, and between the second and third, and again between the third and fourth, there is a small intermediate tooth.
Regarding the Cirri, I have nothing especial to remark.
Geographical Distribution.—I have examined nine sets of specimens, having localities attached to them: three from off New South Wales; two off the Cape of Good Hope; two from the west coast of South America; and two marked only “South Sea.” Hence I am led to conclude that this species is confined to the southern hemisphere, or if it extends into the northern hemisphere, it is probably only in the Pacific Ocean. I do not believe (though so stated in some works) that this species occurs on the shores of Europe. It seems often to be associated with [Tubicinella]. Some specimens thus associated, sent by Mr. Bennett to Professor Owen, were said to have been attached to the Balæna australis.
2. [CORONULA] DIADEMA. Pl. [15], fig. [3], [3 a], [3 b]; Pl. [16], fig. [1], [2], [7].
LEPAS DIADEMA. Linn. Systema Naturæ, 1767.