_Fossil_ in the glacial deposits of Scotland, Uddevalla, and Canada; in the mammaliferous and Red Crag of England; Mus. Lyell, Sowerby, S. Wood, &c. _General Appearance._--Shell conical, somewhat convex; white, sometimes tinted yellowish, from the thin investing membrane; the produced tips of the terga are purple: the parietes of each compartment have from two to four strong, prominent, sharp, straight longitudinal ribs; these are sometimes irregular, and rarely, as will presently be described, they are absent. The radii are smooth and of considerable breadth; their summits are nearly parallel to the basis or only slightly oblique: hence the orifice is entire; it is rather small and ovate, being broad at the rostral end, and very sharp and narrow at the carinal end. _Dimensions._--The largest specimens which I have seen from Great Britain or Ireland, have been 1.3 of an inch in basal diameter: in Mr. Cuming's collection, however, there was one much depressed specimen from the Shetland Islands, 2.1 in basal diameter: a regularly conical specimen from the coast of Massachussetts attained a nearly equal diameter; out of the glacial deposits in the Isle of Bute, Scotland, several specimens had this same diameter, namely, two inches, and were even more steeply conical, being 1.85 in height; some glacial specimens from Uddevalla and Canada, in Sir C. Lyell's collection, were 1.7 in basal diameter. Hence, it appears, as we shall presently see is likewise the case with _B. crenatus_ and _Hameri_, that northern specimens, and those from the United States and from the glacial deposits, often exceed in dimensions those from Great Britain or Ireland. _Scutum_: the lines or ridges of growth are broad and prominent; they are divided into square beads by fine striæ, radiating from the apex: and hence the valve is longitudinally striated. Internally, the articular ridge is extremely little prominent; the adductor ridge, or what must be called such, runs straight down under the articular ridge, making a deep longitudinal pit for the lateral depressor muscle. _Tergum_: the apex is a little produced, and coloured purple, as well as the upper internal surface of the valve; there is no longitudinal furrow, only a very slight depression: the spur is placed close to the basi-scutal angle; it is rather long, and measured across the upper part, is half as wide as the valve: its lower end is truncated and rounded; the basal margin slopes towards it. Internally, a very small portion of the scutal margin is inflected: the articular furrow is shallow and broad: the crests for the depressores are feeble. In young specimens the spur is bluntly pointed.