ARNOLD — THE PALEONTOLOGY AND STRATIGRAPHY OF SAN PEDRO. 155 



Section Rupellaria Fleurian. 



Shell inflated and roundoil in front, attenuated and more compressed behind; sculpture 

 chiefly radial ; stronger anteriorly. 



Type, Petricola lithophncia (Iletziiis). 



95. Petricola (Rupellaria) lamellifera Conrad. 



Venus lamdlifera Con., Jour. Phil. Acad. Nat. Sci., Vol. VII, 1837, p. 257, PI. XIX, tig. 19. 



Vetierupis cordicri \'^r. B, Desh., B. M. Catal. Ven., p. 191, No. i. 



Petricola cordtert Desh., Rev. Zool. Soc. Cur., 1839, p. 358; Mag. ZooL, Pi. XVIII. 



Rupellaria lamellifera Con., Carpenter, Proc. Zool. Soc, 1856, p. 214. Carpenter, Brit. Assn. 

 Rept., 1863, p. 641. Cooper, 7th Ann. Rept. Cal. St. Min., 1888, p. 262. Keep, 

 West Coast Shells, p. 183, fig. 156, 1S92. Wii.i.iamson, Proc. U. S. Nat. Mus., Vol. 

 XV, 1892, p. 184. 



Shell of medium size, convex, equivalve, inequilateral, thick; beaks anterior to middle, 

 marginal, not much elevated, turned slightly forward ; anterior extremity evenly rounded ; posterior 

 extremity widely truncated; surface sculptured by ten or twelve lamelliform, concentric, slightly 

 reflected ribs, and rather faint radiating sulcations; lunule indistinct, sculptured; ligament rather short; 

 hinge strong, with two bifid and one single tooth in each valve; pallial sinus deep, narrow, pointed; 

 muscle-impressions distinct, oval. 



Dimetuions. — Long. 26 mm.; alt. 21.5 mm.; diam. 16 mm. 



This species resembles Petricola cnrditoides, but is distinguishable by the 

 prominent concentric himellar frills. Like P. carditoides this shell bores in the 

 rocks; and all of the specimens obtained were broken from the shale pebbles in 

 tlie upper San Pedro gravels of Deadman Island. 



Living. — Farallon Islands to San Diego (Cooper). 



Pleistocene. — Santa Barbara (Cooper): San Pedro (Arnold). 



Section I'etricolaria Htolicska. 

 Shell elongated, pholadiform, thin; hinge-teeth protracted, slender; pallial sinus deep. 

 Type, Petricola pholadiformis (Lanej. 



96. Petricola (Petricolaria) denticulata Sowerbij. 



Petricola denticulata Sbv., Thes. Conchyliorum {'.). 



Petricola pedroana Con., Pac. R. R. Rept., Vol. V, p. 324, Pi. Ill, fig. 24, 1856. Gabb, Pal. Cal., 



Vol. 11, p. 119, 1869. 

 Petricola denticulata Sv.\., Carpenter, Brit. Assn. Rept., 1856, pp. 244, 297. 

 Cypricardia pedroana CoN., Cooper, Bull. No. 4, Cal. St. Min. Bureau, 1894, p. 25. 



Shell small, elongate-oval, convex, thin; surface sculptured by fine, concentric, incremental 

 lines, and numerous, undulating, radiating ridges, which are most prominent on the anterior ])ortion 

 of the shell; anterior end short and rounded ; teeth small; pallial sinus deep. 



Dimensions. — Long. 20 mm.; alt. 12 mm.; diam. 10 mm. 



