---- ---- Chemnitz. Conch., vol. 8, Tab. 99, figs. 843, 844.
BALANUS DIADEMA. Bruguière. Encyclop. Method., n. 164, fig. 13, 14 (1789).
CORONULA ------ De Blainville. Dict. des Sc. Nat. (1824), Tab. 117, fig. 4.
-------- ------ Leach. Encyclop. Brit. Suppl., vol. iii, 1824.
-------- ------ Chenu. Illust. Conch., Plate, fig. 3.
-------- ------ Burmeister. Beiträge zur Naturgeschichte der Rankenfüsser, 1834, Tab. 2, fig. 1-14, 18.
Shell crown-shaped, with longitudinal convex ribs, having their edges crenated; orifice hexagonal: radii moderately thick, very broad: terga absent or rudimentary.
Hab.—Attached to whales, in the Arctic Seas; United States and Great Britain; Gulf-Stream, Atlantic Ocean; New Zealand(?).
General Appearance.—As previously remarked, owing to the fulness of the generic description, minute details on structure, excepting those characteristic of the present species, need not here be given. The crown-like shape of the shell is well expressed by its name of Diadema, but the crown tends to pass into a cylinder. The radii are extremely broad. The orifice is large, and neatly hexagonal: when the operculum is removed the whole inside of the cup-formed shell can be seen at once, for the flat membranous basis is much smaller than the orifice. The under side of the shell is deeply concave. The outside of each compartment is formed by broad, rounded, and somewhat prominent, rarely divided, ribs (i. e. the transverse ends of the folded walls); these ribs are closely united together by finely serrated lines of junction (Pl. [16], fig. [1], f). Their surfaces outside are delicately striated longitudinally, and plainly crossed (more plainly than in the foregoing and the next species) by irregular, transverse ridges, especially in the lower part of the shell. The largest specimen which I have seen was two and a half inches in diameter and two in height.
Scuta.—These are placed close together at the rostral end of the orifice, and are imbedded in the brownish, tough, longitudinally plicated, horny substance, which extends far beyond both ends of the valves. In outline (fig. [3 b]) they are mitre-shaped, or rounded and sub-triangular, a little curved, and more or less elongated, being most so in young specimens; they are, however, less elongated and rather more massive than in [C. balænaris]. Terga,—these seem entirely absent in most specimens; but in one (fig. [3 a]) I found a rudiment, namely, a short thin plate of shell, barely visible to the naked eye, extending parallel and near to the tergal margin of the scutum. The lips of the aperture of the sack are prominent, and highly so towards the carinal end.