ARNOLD — THE PALEONTOLOGY AND STRATIGRAPHY OF SAN PEDRO. 121 



Pleistocene. — San Pedro (Cooper, Arnold) : San Diego (Arnold). 



Pliocene. — Santa Rosa; Twelve Mile House, San Mateo County; Soquel, Santa 

 Cruz County; San Fernando; San Diego well (Cooper): San Diego (Arnold). 



Miocene. — El Toro Ranch, Monterey County; Foxin's, Santa Barbara County 

 (Cooper). 



Genus Lithophaga Bolten. 



Shell cylindrical, inflated in front, wedge-shaped behind; epidermis thick and dark; interior 

 nacreous. 



Type, Mijtilus lithophagus Linne. 



44. Lithophaga plumula Hanley. 



Lithophagus plumnla Hanley, Carpenter, Brit. Assn. Rept. , 1863, p. 644. Keep, West Coast 



Shells, p. 171, 1892. Williamson, Proc. U. S. Nat. Mus. , Vol. XV, 1892, p. 192. 

 Lilhophaga plumula Hanley, Dall, Trans. Wagner Inst. Sci., Vol. Ill, Part. 4, 1898, p. 799. 



Shell subcylindric, with nearly terminal beaks; two radial sulci extending backward from 

 the beaks, incrustation plume-like, arranged in a distinct pattern on the areas between the sulci, and, 

 when projecting beyond the ends of the valve, apposited symmetrically. 



One perfect cast of a shell of this species was found on the inside of a Metis 

 alia in the upper San Pedro series at Los Cerritos. 



Living. — Monterey to San Diego (Carpenter). 



Pleistocene. — San Pedi'o (Arnold). 



Order ANOMALODESMACEA. 



Superfamily ANATINACEA. 



Family XIII. PERIPLOMID.E. 



Genus Periploma Schumacher. 



Shell oval, very inequivalve, inequilateral, slightly nacreous ; left valve deepest; posterior 

 side very short and contracted; hinge with a narrow, oblique, spoon-shaped process in each valve, 

 and a small triangular ossicle; an internal rib proceeds from under the hinge to the posterior margin; 

 muscular impressions unequal, the anterior long and narrow, the posterior small, semilunar; pallial 

 impression marginal. 



Periploma incequivakis Schum. is a characteristic species. 



45. Periploma argentaria Conrad. 



Periplofna argenlaria CoN., Journ. Phil. Acad. Sci., Vol. VII, 1837, p. 238, PI. XVIII, fig. 8. 

 Carpenter, Proc. Zool. Soc, 1856, p. 211; Brit. Assn. Rept., 1863, p. 638. Cooper, 

 7th Ann. Rept. Cal. St. Min., 1888, p. 258. Keep, West Coast Shells, p. 204, 

 fig- i75> 1892. Dall, mss., 1900. 



( 11 ) December 27, 1902. 



