urfaces of the valves are roughened with small imbricated
points. Exteriorly the valves are covered with yellow membrane, with
rows, corresponding with each zone of growth, of very minute, yellow,
horny spines, generally having their tips bent over, and so made
hook-shaped. These spines are less than 1/600th of an inch in length.

_Scuta_, triangular; internally concave, with a large depression for the
adductor muscle; there is the usual small roughened internal knob, or
tooth, at the rostral angle of both the right and left hand valves.
Tergal margin straight, overlapping about one third of the entire width
of the terga.

_Terga_, irregularly oval, with the scutal margin straight; basal point
blunt, with the two sides placed at about an angle of 45 deg. to each
other; the lower part of the carinal margin, immediately over the latera,
(as seen internally,) is slightly hollowed out. Exteriorly, towards the
bottom of the valve, from the overlapping of the scuta, of the latera,
and of the carina, only a narrow rounded ridge is exposed, which runs
down to the basal angle at about one third of the entire width of the
valve, from the scutal margin. Internally the valve is slightly concave.

The _Carina_ slightly overlaps the terga; internally concave; generally
with a large upper portion freely projecting; inwardly curved, without
any central crest or ridge; valve nearly as wide as the middle part of
the terga; inner growing or corium-covered surface, with its basal
margin, protuberant and arched.

_Rostrum_ (Pl. VIII, fig. 1 _a'_, _a_, and greatly magnified 1 _b'_)
very narrow; rarely more than two or three layers of growth are
preserved; the sides are deeply sinuous, owing to each zone widening
downwards; basal margin rounded; in width equalling about two and a half
of the uppermost scales of the peduncle, and about half as wide as the
latera.

_Latera_, small, placed obliquely, and parallel to the lower carinal
margin of the terga; longer axis equal to five of the uppermost scales
of the peduncle, and to nearly half the width of the base of the carina;
growing surface (or a section made parallel to the growth-layers,) is
narrow, elliptic, pointed at both ends, but the carinal half rather
thicker than the scutal half.

The _Peduncle_ varies in length, generally about twice as long as the
capitulum, in one specimen above thrice as long. The upper part as wide
as the capitulum, the lower part sometimes much attenuated.