Shell moderately heavy. from 25 to 40 mm. in length, conical, elevated and narrower anteriorly. Front slope straight or concave, posterior convex. Base ovate or sub-oblong. Apex anterior to shell center, more or less curved forward, and with the orifice placed immediately before it. The latter is key-hole shaped, the rear wall thick¬ened and sometimes excavated obliquely, the sides frequently projecting upwards to form one prominent tooth which sometimes divides into two, while frontally and laterally it may sometimes have a very tine black line on the outside. Its length is from one-sixth to one-ninth that of the shell. Surface sculptured with numerous close set radiating sharp ribs, every fourth one larger. Concentric overlapping growth laminae cross the shell forming small scales where the ribs are intercepted. Color very variable: may be unicolored milky white, cream, buff, light pink or dark gray. With any of these colors as a ground, there may be eight or nine gray or brown rays which are solid, in broken lines, or in scattered spots; with the same ground color there may be a broad basal band of dark gray or black. Margin sharply and finely crenulated. Interior color white or bluish gray, the color pattern and sculpture of the outside showing through. The internal callus of the orifice is of the same color as the rest of the inside. It is sharply truncated and excavated obliquely, forming a deep pit behind: sometimes it is encircled by a black line. .Muscle sear slightly impressed, narrower anteriorly.

Diodora cayenensis (Lamarck, 1822) Cayenne Keyhole Limpet
Distribution: Maryland to Florida, Texas to Brazil; Bermuda. Size: 25 to 51 mm (1 to 2 inch).
Description: Color light brown to greenish, usually with dark radiating rays; shell shape conic, moderately heavy, elevated, and narrow anteriorly; sculpture of radial and spiral ribs, each fourth radial rib large with 3 smaller radial ribs in between on mature specimens; a pit is formed behind the callus of the orifice; anterior slope straight, posterior slope slightly convex; orifice keyhole shaped, in front of and lower than apex. Habitat: In Texas found on shelly bottoms and rocks from intertidal region to a depth of about 35 m (115 ft). Occasional to common on jetties. Depth range 0 to 143 m (469 ft).
Remarks: Shape and height of shell variable. In Texas the maximum size recorded is 27 mm (about 1 in). See Perez-Farfante (1943a); Ode and Speers (1969b); Tunnell and Chaney (1970); Rehder (1981); Redfern (2001).
Synonym: D. alternata Say, 1822.

This is an exceedingly variable species, the most variable characters being the general shape of the shell and the coloration. The color pattern has probably been chiefly responsible for the variety of names applied to this species. The very large collection of material at hand indicates that it is impossible to draw any line among these different forms. Certain specimens possess two sets of color characteristics each one of which has had a name applied to it.
It is exceedingly unfortunate that the well known name of alternata Say must be replaced by an earlier one proposed by Lamarck; both appeared in 1822. but that of Lamarck was published in May while that of Say was published in June.

Range. Maryland and south, including the Gulf of Mexico, the West Indies and South America, to Brasil. D. cayenensis is found from the intertidal realm to depths of about 20 fathoms. It occurs mainly on rocks.