_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 early stages of growth, (as explained under _S. vulgare_,) and, therefore, they form a depressed rim. _Rostral Latera_, almost exactly triangular, curved; basal margin furnished with a just perceptible rim. _Infra-median Latera_, quadrilateral, sides unequal in length; the carino-basal margin being the longest; in area not quite twice double the rostral latera; directed obliquely upwards. _Carinal Latera_, sub-triangular, produced upwards, with the apex rounded, and the two lateral margins hollowed out; the basal margin exceeds a little in length the basal margin of the rostral latera. The umbones of these two latera are seated at their basal outer angles, so that the growth of the valves is towards each other and upwards. The umbo of the infra-median latus is seated at the baso-rostral angle, and hence the growth is obliquely upwards. The umbones of the rostral latera must have been close together, over the unknown rostrum. _Length_ of capitulum about 4/10th of an inch. _Peduncle_, only small fragments are preserved; the calcified scales are small, closely imbricated, several of them together only equalling in length the basal margin of the rostral latera. Each scale is thin, transversely elongated; basal imbedded portion straight; upper margin rounded. _Mouth._--Labrum with the upper part highly bullate, forming an overhanging projection; palpi apparently small and narrow. _Mandibles_, narrow, produced, with three teeth; inferior angle pectinated, as is sometimes the third tooth; the distance between the tips of the first and second teeth equals that between the second tooth and the inferior angle. _Maxillæ_, extremely narrow, produced, without any notch; spinose edge exactly one third of the length of the mandibles: beneath the two upper great spines there are only three or four pair of spines; on the convex upper margin there are some minute tufts of the smallest hairs. _Outer Maxillæ_, rounded with the inner margins very sparingly but continuously covered with bristles. I could not ascertain whether the olfactory orifices were tubular.