Shell small for the subgenus, thin, markedly depressed, ends of shell slightly elevated. Fissure forward of center, its posterior termination at about the midpoint of the shell, length of fissure about 4 times the width; fissure tripartite, constricted at both ends, walls of fissure sloping inward when viewed exteriorly; length of shell 5.5 times length of fissure. Surface of shell nearly smooth, sculptured with extremely fine radial grooves and fine concentric growth lines, appearing finely beaded under magnification. The termination of the grooves project slightly at the margins. Color purplish-brown with white rays; interior yellowish-green, glossy, transmitting the external color pattern. Length, 13.2, width, 9.5, height, 2.6 mm.

Fissurella (Clypidella) morrisoni McLean, 1970. The ends of the shell are slightly elevated, the anterior end narrowed. The orifice is greatly elongate, and the radial ribbing is extremely fine. Outside, the shell is rayed with white and purplish brown; the inside is light green, with the external color pattern showing through. Length, 16 mm; width, 11 mm; height, 3 mm. Four specimens are known from Bahía Honda and San José Island, Pearl Islands, Panama.

Fissurella morrisoni is referable to the subgenus Clypidella Swainson, 1840, distinguished by its extremely depressed shell, raised ends and elongate fissure, and on these characters differs widely from the tropical eastern Pacific fissurellas in the subgenus Cremides H. & A. Adams, 1854. Fissurella morrisoni differs from the 2known Caribbean species of Clypidella, F. fascicularis Lamarck. 1822, and F. punctata Fischer, 1857 (see Farfante, 1943, p 12), in having extremely fine ribbing, rather than the coarse ribbing of the 2 Caribbean species. The species is dedicated to Dr. Joseph P. E. Morrison of the Division of Mollusks, U.S. National Museum, who has been most helpful during my visits to that institution, and whose industrious collecting in 1944 produced sufficient specimens of this remarkable species to warrant its description.

Type Locality: Bahia Honda, Panama, 7° 44' 25" N, 81° 32' 45" W, intertidal in rocky area, R/V Velero HI station 861-38, 1 March 1938, 1 specimen.
Type Material: Holotype, LACM-AHF 1374.
Referred Material: Three lots, single beachworn specimens USNM 588191, USNM 588476, USNM 587950, all from San Jose Island, Perlas Islands, Panama, collected by Dr. J. P. E. Morrison, March, July and August, 1944. The largest of these, USNM 587950, measures: length, 16.2, width, 11.1, height, 2.6 mm.