PANAMIC-PACIFIC PELECYPODA 305 



P. staminea (Conrad), a common West Coast species ranging north- 

 ward to Alaska, has been recorded from Cape San Lucas at the southern 

 tip of Lower California. The species resembles P. thaca by its surface 

 sculpture and chalky or drab color but differs in the absence of an escutcheon 

 and in hinge details. The species should probably be transferred to 

 Callithaca to which it appears more closely related. 



Callithaca Dall, 1902 (type species Tapes tenerrima Carpenter) has 

 been considered a section or subgenus of Protothaca, but a lunule and es- 

 cutcheon are lacking in the type species. 



Subgenus TROPITHACA, new subgenus 



Type species Protothaca grata (Say). 



Shell small or of medium size, elongately ovate, convex, the umbones 

 and beaks at the anterior one-fourth. Hinge of Protothaca. Escutcheon 

 much reduced in size. Color of surface extremely variable, sometimes quite 

 plain or marked with radial rays and zigzagged lines, the interior white or 

 irregularly stained with violet. PaUial sinus short. 



A group of small tropical or warm-water species distinguished from 

 typical Protothaca by their more colored surface, smaller size, and simpler 

 sculpture which is not so sharply divided into zones. Shells are diversely 

 patterned, hardly two specimens marked alike. 



Protothaca (Tropithaca) grata (Say) Plate 53, figures 2-2b, 7 



Venus grata Say, 1830, Amer. Conch., No. 3, pi. 26, (three figures) West coast of 



Mexico. — Reeve, 1863, Conch. Icon., vol. 14, Venus, pi. 3, figs. 8a, 8b. 

 Venus fuscolineata Sowerby, 1835, Proc. Zool. Soc. London, p. 41 Guacomayo. — Reeve, 



1863, op. cit., pi. 16, fig. 69. 

 Venus tricolor Sowerby, 1835, op. cit., p. 41 Puerto Portrero. — Reeve, 1863, op. cit. 

 Venus discors Sowerby, 1853, op. cit., p. 42 Santa Elena and Guacomayo. — Reeve, 



1863, op. cit., pi. 7, figs. 22a, 22b. 

 Tapes fuscolineata (Sowerby), 1853, Thes. Conch., vol. 2, Tapes, p. 698, No. 69, pi. 



151, figs. 145. 

 Tapes tricolor (Sowerby), 1853, op. cit., p. 699, pi. 151, fig. 153. 

 Tapes discors (Sowerby), 1853, op. cit., p. 698, figs. 148-150. 

 Protothaca {Callithaca) grata (Say), Hertlein and Strong, 1948, Zoologica, vol. 33, 



pt. 4, p. 193. 



The above synonym could be greatly extended but the principal names 

 given to this common species are noted. 



The shell is of medium size, elongately ovate, the beaks nearer the 

 anterior side, and the surface covered with small, closely spaced, radial 

 cords, in places finely reticulated by concentrics. Coloration is varied, us- 

 ually in tones of white, yellowish gray and purplish black, arranged in 

 broad rays, blotches or lines, two specimens seldom the same. The interior 

 may be white or with blotches of violet or purple. The sculpture is formed 

 by numerous, low radial riblets, those on the anterior and posterior slope 

 concentrically noded, the ribs on the middle plain. The lunule is small, 

 cordate, and sculptured with radial lines. The escutcheon is absent or 

 narrow, usually the posterior-dorsal margin is narrowly flattened distally, 

 the end of the right valve noticeably overlapping the margin of the left 

 valve. Hinge typical of the genus, the pallial sinus short but distinct. 



