Cerion (Umbonis) stevensoni Dall, 1900

Original Description

"Shell very variable in general form, but in general roughly cylindrical, with the nuclear whorls as it were jammed down into the blunt summit of the cylinder, with the base carinate at the periphery, where the ribs cease, and below that constricted; whorls 8-10, nuclear ones nearly smooth, gradually developing fine transverse ribbing with subequal interspaces; these become stronger, with a strong revolving thread behind the suture; at the third whorl then the diameter of the shell suddenly increases the sides develop strong transverse rather irregular ribbing with wider interspaces, the ribs extending from the suture to the basal keel, beyond which they rarely extend; the base beyond the keel is constricted, rudely transversely wrinkled, inside the verge of the umbilicus centrifugally impressed and axially deeply perforate; aperture very variable in shape, with a broad, flattish, rather thin reflected margin; there is a parietal short lamina centrally situated and strong, but no trace of an axial fold; color light brownish or ashy to white, the whole surface sharply spirally striated, the striae sometimes crowded, sometimes distant." [Dall, 1900: 65].

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