l
species, but run up a little way along the tergal margin. The umbo is
seated at the uppermost point, and, therefore, the main growth is
downwards. There is a large rounded depression for the adductor muscle
(_a_, fig. 2 _a'_), and higher up, opposite the tergo-lateral angle,
there is another hollow (_b_), for the lodgment of the males; this
latter is of nearly the same shape as the hollow for the adductor
muscle, but rather more conspicuous than it. From the appearance of the
under surface of the scuta, it might readily have been thought, that
there had been two adductor muscles.

_Terga_, of large size, longer than the scuta, flat, triangular, with
the whole inferior part much produced and spear-like. A portion of the
apex, must have projected freely above the sack.

_Carina_ (Pl. VI, fig. 2 _b'_), simply bowed (_i. e._, not rectangularly
bent), with the umbo (and primordial valve) seated at the upper point;
rather massive, narrow, only slightly increasing in width from the upper
to the lower end; the two sides are flat, and at right angles to the
roof, which is bordered on each side by a rather broad, square-topped
ridge (_see section_ fig. 2 _c'_), or the roof may be said to have a
square-edged furrow running from the apex to the basal margin, and
widening downwards; these two ridges have their lines of growth oblique,
and hence have a twisted appearance; the central depressed portion of
the basal margin, which is square or truncated, descends lower down than
the two ridges. The sides of the valve close to the apex are broad, and
consist, as I believe, of intra-parietes, as well as of parietes, but
these parts are not separated from each other by ridges, as is commonly
the case, more especially with the fossil species. I have described the
carina in some detail, on account of its resemblance to that of the
cretaceous _S. fossula_, _S. trilineatum_, and _S. quadricarinatum_.

_Rostrum_, unknown; but one probably existed.

_Upper Latera_, of large size, elongated, quadrilateral, approaching to
diamond-shaped, with the angles rounded, nearly twice as long as broad;
almost flat; upper half acuminated, lying between the scuta and terga;
the lower half broad, forming a rectangular projection lying between two
latera of the lower whorl. The umbo is near the apex, the greater part
of the growth being downwards, but the valve is added to a little, round
the two sides of the apex; these additions do not take place in the