ARNOLD — THE PALEONTOLOGY AND STUATIGKAi'lIY OF SAN PEDRO. 121 



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



Pliocene. — Santa Rosa; Twelve Mile House, San Mateo County; So(]Ucl, Santa 

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



Miocene. — El T(no llancli, INIonterey County; Foxin'.s, Santa Barbara County 

 (Cooper). 



Genus Lithophaga Boltcn. 



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

 nacreous. 



Type, Mijtilus lUhojjhagus Linne. 



44. Lithophaga plumula II unity. 



Lilhophagus plionula Hanley, Carpenter, Brit. Assn. Rept. , 1863,0. 644. Keep, West Coast 



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

 Lithophaga 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 iiLside of a Metis 

 alia in the upper San Pedro series at Los Cerritos. 



Liviny. — Monterey to San Diego (Carpenter). 



Pleistocene. — San Pedro (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 incequicalvis Schuiu. is a characteristic species. 



45. Periploma argentaria Conrad. 



Periplmna argentaria 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. 25S. Keep, West Coast Shells, p. 204, 



fiy- '75' 1892. Dall, mss., 1900. 



I u I December 27, l!IU2. 



