iption._--Capitulum much flattened with the apex produced, of a
pale brown colour, sometimes faintly tinted purple, composed of 14
valves, of which the rostrum is rudimentary and barely visible
externally; valves thin, white, translucent, smooth, slightly marked by
the lines of growth, separated from each other by rather wide
interspaces of colourless membrane, which is thickly clothed by small,
articulated spines of unequal length. The valves, excepting sometimes
their umbones, are also covered with membrane, bearing spines, placed in
rows parallel to the lines of growth; the spines are particularly
numerous round the orifice of the sack.

_Scuta_ slightly convex, thrice as long as broad; upper part much
acuminated; occludent margin almost straight; basal margin nearly at
right angles to the occludent margin; the tergal margin is separated
from the lateral margin by an angle more or less prominent; a slight
curved ridge runs from the umbo to this angle, and this deserves
especial notice, inasmuch as it indicates the outline which the valve
assumed in its earliest growth, and which is permanently retained in
most of the older fossil species. Along the occludent margin, there is a
trace of a ledge, developed in a variable degree, and which is noticed
only on account of the plainly visible ledge along this same margin, in
the allied genus Oxynaspis. The umbo, or centre of calcification, is
seated close to the occludent margin, and at about one fourth of the
length of the valve from the apex. Internally, (fig. 15, _a'_, Pl. V,)
the part above the umbo is flat; and beneath this upper part, there is a
large rounded hollow (_d_) for the adductor muscle: a fold or
indentation (_a_) running downwards from the umbo, extends in a very
oblique line across the occludent margin. This fold is of high interest
as giving lodgment to the Complemental Males, and will hereafter often
be referred to.

_Terga_, triangular, flat; occludent margin, very slightly arched.

_Carina_ much bent, with the umbo placed at barely one third of the
entire length of the valve from the apex. Two very slight ridges can be
perceived, one on each side, running from the umbo to the basal margin,
and separating the roof from the parietes of the valve; these ridges are
of great use in distinguishing the fossil carinae of Scalpellum, from the
carinae of Pollicipes. The part above the umbo is formed by the upward
production of a marginal slip along each side of