ARNOLD — THE PALEONTOLOGY AND STRATIGRAPHY Ol' SAN PEDRO. 179 



Pleistocene. — Santa Barlmra to f^an Diego (Cooper): f^an Pedro; San Diego 

 (Arnold). 



Pliocene. — Santa Barbai'a (Cooper). 



Superfamily MYACEA. 

 Family XXXVT. MYACTDyE. 



Genus Platyodon Conrad. 



Shell ventricose, with concentric, undulating strife, and a small groove from the apex to the 

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

 bi-emarginate. 



Type, Platijodon cancellatus Conrad. 



131. Platyodon cancellatus Conrad. 



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



Platyodon cancellatus 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 strict, 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 Myacidm, this species has somewhat the sliape of 

 a Pliolas, 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. 



Rather rare in tlie 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 striae; right 

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

 or obsolete. 



Type, Cryptomya californica Conrad. 



