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124 Lamarck. The latter, however, is a distinct species from the West Indies which is characterized by a splotch of bright orange-red around the umbilical region (see pl. 3l).

1 to $1 1/2$ inches in length, $3/4$ as wide. Characterized by its sharp-angled spire, flat sides, white to cream color, and by the numerous, long, wavy, weak, axial ribs. Base of shell with 5 to 8 small, finely fimbriated, spiral cords, and a small ridge at the base of the columella which has about a dozen small axial ridges. Commonly found under rocks at low tide on the Lower Florida Keys. Operculum variable, but usually thick, convex and with a small or large dimple.

The subspecies, imbricata Gmelin, from the West Indies has stronger, longer and fewer axial ribs which extend to the flat base of the shell and are hollow at their ends. The subspecies guadeloupensis Crosse from the Greater Antilles is intermediate between these two. Both moderately common at low water.

2 to 3 inches in length and width. Similar to A. tuber, but with 9 to 10 spiral rows of numerous, hollow, scale-like spines on the lower$2/3$ of the last whorl, 5 of which are on the base of the shell. Operculum thick, convex, and finely pustulose. Moderately common in the West Indies.

1 to 2 inches in length, equally wide. Characterized by the peculiar green-and-white, cross-hatched color scheme, by the low, blunt, smooth axial ridges, and by the smoothish base of the shell. Sometimes mottled in soft browns. Common below low water in the West Indies, rare in Florida. Operculum with a thick, arched, tapering ridge on the exterior (like a large comma).