Scuta largely overlapping the terga. Carina with a slight central internal ridge in the upper part. Latera with their internal surfaces symmetrically and broadly oval, more than one third of the width of the carina. Terga with the basal points narrow, and the carinal angle produced. Rostrum and peduncle unknown.

Mandibles, with four times as many pectinations between the first and second main teeth, as between the second and third; distance greater between the tips of the first and second teeth, than between the tip of the second tooth and the inferior angle. Maxillæ widely notched, with the inferior part forming two obscure prominences.

Hab. unknown. Imbedded in a massive coral. Brit. Mus.

The specimens are in a rather bad condition, and have been disarticulated. They are of rather small size; the rostrum and peduncle are lost, and animal’s body much injured.

Valves white, thin, translucent; teeth on the projecting rims small, narrow, standing further apart than their own width. The upper layers have undergone but little disintegration or scaling off, and consequently the carina and terga project freely. The valves, where not rubbed, are covered by bright yellow membrane, which is thickly clothed with rows of spines; these are small on the exterior surfaces, but are very large and hooked in certain parts, as near the tergal margins of the scuta, and on the carinal margins of the terga, and especially on the inner face of the upper free part of the carina. Here the hooked spines ([fig. 4 d]) are trifid or quadrifid, and are very conspicuous.

Scuta, as seen externally, triangular; they overlap half the width of the terga; on their internal faces ([fig. 4 a]), in the upper projecting part, there is a strong ridge, against which the scutal margin of the terga abuts. There is a deep and conspicuous pit for the adductor muscle.

Terga, as seen externally, nearly triangular. The ridge which leads from the apex to the basal angle, is rounded, central, and extremely prominent; but does not form a furrow, or include the overlapping margin of the scuta. The basal angle is narrow, spur-like, and slightly hollowed out on both margins. The growing corium-covered surface ([fig. 4 b]) is transversely elongated, with the occludent margin rounded, and the carinal angle much produced, but not forming a roughened knob.

Carina ([fig. 4 d]), concave within, with a slight central ridge in the upper free portion. The inner growing surface is concave, almost pentagonal, with a just perceptibly raised central rim in the upper part, and with two minute prominences on each side, against which the produced carinal angles of the terga abut.

Rostrum, lost.

Latera ([fig. 4 c]), growing surface (or a section parallel to the growth-layers,) symmetrically oval, more than one third as wide as the basal margin of the carina. Several zones of growth preserved.