Shell smooth, gray or dirty white: radii broad, smooth-edged: scutum without an adductor ridge; tergum with the spur distinct from the basi-scutal angle: scutum and tergum sometimes calcified together.
Hab.—Tierra del Fuego, Falkland Islands, Chiloe. Attached to tidal rocks and sometimes to floating timber; Mus. Brit., Darwin, &c.
General Appearance.—Shell fragile, either steeply conical with a large orifice, or sub-cylindrical; surface smooth, grayish or white, with large portions covered by pale brown epidermis. Radii broad with their summits oblique, smooth, slightly arched, exhibiting a large surface of the alæ. The alæ usually have their summits much less oblique than those of the radii; the portion added during diametric growth is of a dead white colour. The growth ridges on the scuta are very little prominent, and are crossed by a very obscure band of blueish-gray. The largest specimen which I have seen was .8 in basal diameter, and the longest cylindrical variety .55 of an inch in height.
The Scuta are remarkable for not having any adductor ridge or crests for the depressor muscles; the articular ridge is prominent, but it is short, not extending down half the valve. I have mentioned under the genus, that in many specimens at the Falkland Islands the scuta and terga were calcified together.
The Terga are rather small: the basal margin on the carinal side of the spur is always hollowed out, but to a very variable degree, as may be seen in the three figures ([6 c]-[6 e]); this margin is generally dentated with one or two little points; and an inner lamina of shell sometimes depends beneath the outer lamina, to which the opercular membrane is attached, as may be seen in the figure ([6 d]) of the external surface of the valve. The crests for the depressor muscles are well developed. The tergal margin is broadly inflected, and the articular ridge prominent, making the articular furrow deep. The spur is rather narrow, and is either ([6 d], [6 e]) bluntly or sharply pointed. The basal margin on the scutal side of the spur, is hollowed out, but to a variable depth.
Structure of the Parietes and Radii.—The parietes are thin; at their internal basal edges they are finely striated in longitudinal lines. The radii are solid, with quite smooth edges; they are generally covered by the epidermis. The sutural edges of the alæ are likewise smooth, these are added to largely during the diametric growth; and their summits, as already stated, are much less oblique than the summits of the radii. The internal surface of the shell is smooth, and is tinted pale dull purple. The lower edge of the sheath can hardly be said to be free. The carinal margins of the compartments project a little inwards.
Mouth: the labrum is deeply notched, and supports five little teeth on each side; the palpi are thickly clothed with spines on their inner sides; the mandibles have five or only four teeth: the maxillæ are notched, and the outer maxillæ bilobed.
Cirri: the first pair has one ramus nearly twice as long as the other. The three posterior cirri are elongated, and each segment supports five or six pairs of long spines, with a few minute intermediate bristles.