Structure of Shell and Radii.—Owing to the shell not spreading much at the base, new folds in the walls are much seldomer formed, and therefore the external longitudinal ribs (i. e. the terminal transversely elongated loops), are much seldomer divided, than in [C. balænaris] or [reginæ]; even rather large specimens sometimes having only the original eighteen folds.[125] When new folds are formed, only one is formed on one, viz., the rostral, side of each suture, instead of on both sides, as in [C. balænaris]. The inner ends of the folded walls, surrounding the basal membrane, are narrow, instead of being almost square, as in [C. balænaris]. The lower edge of the sheath, which projects freely, descends almost to the level of the basal edges of the walls. The outer ends of the folded walls, forming the transverse loops, are internally filled up solidly by calcareous matter, instead of by septa forming tubes, as in [C. balænaris]. The radii (Pl. [16], fig. [2]) are a little thicker in the lower and outer than in the upper and inner part of each compartment; in the middle, they do not reach the sheath by about half the thickness of the compartment, and consequently they are separated from the plates (c′′ in fig. [1] and [7]) on which the alæ rest, by large chambers (v in fig. [7]), which extend up to nearly the apices of the compartments: the extent, however, to which the upper ends of these chambers have been solidly filled up, varies a little. The sinuous plates forming the main portion of the compound radii are rather thinner and closer together than in [C. balænaris]. The alæ are thick, being thickest in the middle part, and there equal the radii in thickness; their lower margins are very short compared with their upper margins, hence they are almost wedge-formed.
[125] These should be counted on the under side of the shell, for if counted from the outside, the number would be thought to be twenty-four, as on the side of each compartment bordering the radius, a half-fold has the appearance of being a whole fold, so that in appearance six folds are added to the real number of the folds. This caution is necessary whatever the number of real folds may be, that is, whether or not the original eighteen folds have been increased by subsequent plications. These remarks, also, are applicable to the other species.
Mouth.—The teeth and fine hairs on the labrum are sometimes obscure, and sometimes plain: close outside the bottom of the medial notch, there is a small hard prominence. The palpi are broad; on their basal exterior margins there is a short row of spines, which do not equal in length the width of the palpi, and therefore are not so long as in [C. balænaris]. The mandibles have five main teeth, of which the second and third show only an obscure rudiment of being double; between these two teeth, and between the third and fourth tooth, there is a small intermediate tooth: the inferior angle is narrow, rounded, and spinose. The extremity of the apodeme of the maxillæ is expanded.
Cirri.—I have only to remark, that the pedicel of the first cirrus is extremely broad, and that the rami are set on in an unusually crooked manner: the basal segment of the shorter and broader ramus of this cirrus has its dorsal surface produced into a plate fringed with very fine hairs.
Geographical Distribution.—I have received only four specimens with certain localities attached to them: namely, the Arctic Seas of Scandinavia; the coast of the United States, and of Britain; and the Gulf-Stream. There is also a specimen in the British Museum, sent by Mr. Stephenson, mingled with shells of Mollusca from New Zealand; but a [Coronula], procured from a floating whale in the early part of the outward voyage, might so easily be sent home with specimens subsequently collected in another county, that I do not as yet fully admit that this species is an inhabitant of the Southern Pacific Ocean: I am less willing to admit this, from suspecting that [C. reginæ] in the Pacific, replaces the [C. diadema] of our Northern Seas.
3. [CORONULA] REGINÆ. Pl. [15], fig. [5]; Pl. [16], fig. [4].
Shell globulo-conical or depressed, with longitudinal, much flattened ribs, having their edges crenated, and their surfaces striated and granulated; orifice hexagonal: radii thin, not exceeding one fifth of the thickness of a compartment: terga absent.
Hab.—Attached to whales, Pacific Ocean; Mus. Cuming, Stutchbury, and Darwin.