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96 $3/4$ to 1 inch in length, moderately strong. Apex small, elevated and hooked over toward the anterior end. Behind it is a small, elongate slit penetrating through the shell. Internally the slit is separated from the apex by a calcareous, convex shelf. Exterior with 14 to 23 major ribs and with 1 to 5 smaller radial ribs between the main ones. The fewer the ribs, the stronger they are. Shell dull-gray externally, glossy-white inside. The border is crenulated. Found at low tide in Alaska and dredged from 20 to 75 fathoms off southern California.

$1/2$ to $3/4$ inch in length, similar to cucullata, but with an almost smooth basal edge; with numerous, much finer radial ribs, and with the internal shelf behind the slit reinforced by a second, straight shelf. Commonly dredged in mud from 10 to 75 fathoms.

Keyhole Limpets with the internal callus of the hole truncated and frequently minutely excavated behind; shell with its basal margin never raised at the ends. Central tooth of the radula wide. Compare with Fissurella. Diadora is a misspelling.

1 to 2 inches in maximum diameter. Orifice just in front of and slightly lower than the apex. Many radial ribs with each fourth one larger. Color variable from whitish, pinkish to dark gray. Interior white or bluish gray. Just behind the callus of the orifice on the inside there is a deep pit. D. listeri is much more coarsely sculptured. A common intertidal to moderately deep water species. It was named by Thomas Say one month after Lamarck’s description as D. alternata.