PANAMIC-PACIFIC PELECYPODA 423 



2. Valves more variable in shape, the anterior side usually shorter and 

 expanded, the surface sculptured by irregular growth lines. The chon- 

 drophoral plate large. 



Genus Sphenia 

 Genus CRTPTOMTA Conrad, 1848 



Type species by monotypy, Sphenia californica Conrad. California. 



Shell small or of medium size (25-30 mm.), ovate, subequivalve, the 

 umbones somewhat flattened and ending in small, inconspicuous, median 

 beaks, the valves depressed to moderately convex, white. Hinge edentulous, 

 the ligament as in Mya, the left valve bearing a large, projecting chondro- 

 phore to which the resilium and tensilium are each attached to a separate 

 scar; in the right valve, the attachment scars of the resilium and tensilium 

 lie in the roof of the umbonal cavity under the beak. The adductor scars 

 are of subequal size, connected across by a simple pallial line. Surface 

 marked with concentric growth lines only. 



Cryptomya has the generic characters of Mya but is smaller and has 

 no pallial sinus. 



Cryptomya californica (Conrad) Plato 77, figures 2, 2a 



Sphaenia californica Conrad, 1837, Jour. Acad. Nat. Sci. Philadelphia, vol. 7, p. 234, 



pi. 17, fig. 11. 

 Cryptomya californica Conrad, 1848, Proc. Acad. Nat. Sci. Philadelphia, vol. 4, p. 121. — 



Grant and Gale, 1931, Mem. San Diego Soc. Nat. Hist., vol. 1, pp. 417-419, 



pi. 21, figs. 7, 8a, 8b, 11, 14a, 14b. 

 Mya californica (Conrad), Sowerby in Reeve, 1875, Conch. Icon., vol. 20, ^fya, pi. 1, 



sp. 3. 



Several other names have been given to this species. For complete 

 synonym see Grant and Gale. 



Shell small or of medium size, the length seldom over 30 mm., ovate, 

 depressed to slightly convex, the umbones flattened, median in position, 

 the beaks small and adjacent. There is a low, obscure posterior-umbonal 

 angle which defines a narrow, depressed posterior-dorsal slope, the anterior 

 end of the shell widely rounded, the posterior crudely, obliquely subtruncate. 

 Surface white to chalky, irregularly marked with concentric growth lines. 

 There is no true pallial sinus, the pallial line assuming a more vertical 

 direction at the posterior end. Adductor scars subequal and generally 

 prominent. 



Length 28.2 mm., height 20 mm., diameter 11.4 mm. Negritos, Peru. 



This is a widely ranging species, especially common in northwestern 

 Peru. 



Range — Alaska to northern Peru. Also in Japan. Ecuador: Santa Elena. 

 Peru: Tumbez; Zorritos; Boca Pan; Mancora; Lobitos; Negritos; Paita; 

 Bayovar. 



Genus PLATTODON Conrad, 1837 



Type species by monotypy, Mya (Platyodon) cancellata Conrad. Re- 

 cent, California to Washington. 



Relatively large, lediform, solid, the anterior side longer, convex and 

 expanded, the posterior side shorter and narrowly produced, gaping. Liga- 

 ment like that of Mya, the chondrophore somewhat smaller. Surface white, 

 often earthy, sculptured with close-set concentrics or emphasized growth 



