ARNOLD— THE PALEONTOLOGY AND STRATIGRAPHY OF SAN PEDRO. 179 



Pleistocene. — Santa Bail)ara to San Diego (Cooper): San Pedro; San Diego 

 (Arnold). 



Pliocene. — Santa Barljara (Cooper). 



Superfamily MYACEA. 



Family XXXVI. MYACID.E. 



Genus Platyodon Conrad. 



Shell ventricose, with concentric, undulating strirc, and a small groove from the ape.x to the 

 ventral margin; posterior side short, radiately striated; spoon-shaped cardinal process dilated and 

 bi-emarginate. 



Type, Platyodon cancellatui^ Conrad. 



131. Platyodon cancellatus Conrad. 



Mya cancellatus Cox., Jour. Acad. Nat. Sci., Phil., Vol. VII, 1837, p. 236, PI. XVIII, fig. 2. 



Platyodon ca7icella(us Con., Cpr., Brit. Assn. Rept., 1863, p. 637. Tryon, Syst. Conch., Vol. Ill, 

 PI. CVI, fig. 28, 1884. Cooper, 7th Ann. Rept. Cal. St. Min., 1888, p. 260. Keep, 

 West Coast Shells, p. 208, fig. 177, 1892. Williamson, Proc. U. S. Nat. Mus., 

 Vol. XV, 1892, p. 183. Dall, Trans. Wagner Inst. Sci., Vol. Ill, Part 4, 1898, p. 858. 



Shell of medium size, subelliptical, ventricose. thin; surface sculptured by numerous fine, 

 concentric lamince of growth, and anterior portion with numerous fine radiating stri.-e, which are 

 obsolete on the anterior portion; a slight furrow extends from the beak to the base; umbo promi- 

 nent; anterior end truncated and gaping; posterior end rounded; cardinal tooth very erect, dilated, 

 bi-emarginate. 



Dimensions. — Long. 49 mm.; alt. 28 mm.; diam. 22.5 mm. 



Although belonging to the Myacidce, this species has somewhat the shape of 

 a Pholas, but may be distinguished by its sculpture. Most of the specimens found 

 were in pairs, and in a fine state of preservation. Specimens identified by Dr. Dall. 



Eather rare in the upper San Pedro series at Deadman Island, San Pedro, 

 Los Cerritos, and Crawfish George's. 



Living. — Bolinas Bay to San Diego (Cooper). 



Pleistocene. — ? Santa Cruz (Cooper): San Pedro (Arnold). 



Genus Cryptomya Conrad. 



Shell inequilateral, transverse, oblong, gaping behind; valves with concentric stri;e; right 

 valve with a lamellar tooth; left valve with a broad fosset; ligament internal; pallial sinus absent 

 or obsolete. 



Type, Cryptomya calif or nicn Conrad. 



