ARNOLD — THE rALEONTOLOC Y AND S TRATIGKAPHY OV SAN PEDRO. IIU 



Shell of incdium size, wedge-shaped, rounded behind, ihin; surface smooth, except for 

 concentric, incremental lines; unibones terminal; dorsal margin slightly depressed in middle; hinge- 

 teeth minute. 



Dimensions. — Long. 55 mm.; lal. 24 mm.; diam. iS mm. 



The only tlillereiice between the Pleistocene .specimens and the living form 

 is in the coloration of the epidermis, tl)at of the former being reddish brown, wdiilc 

 I he latter is bine. Specimens identiiied by Dr. Dall. 



Rare in the lower San Pedro series of Deadman Islanti; and in the npper 

 San Pedro series of Deadman Island, Crawfish George's, and San Pedro. 



Living. — San Pedro (Williamson): Monterey, north; .Iaj)an; circumpolar 

 (Cooper): Atlantic Ocean south to North Carolina (Dall). 



Pleiatocene. — Beiiicia, Solano ('onnty (Cooper): San Pedro (Arnold): Atlantic 

 coast from Lal)rador to St. John's River, Florida; northern Europe; northwest coast 

 of America (Dall). 



Pliocene. — Red Crag, Great Britain (Dall). 



Genus Septifer Reduz. 



Shell ecjui valve, very inequilateral; ventral niaigin subconcave and cut out for the passage 

 of the byssus; beaks subterminal, curved; hinge without teeth, furnished with a lamellar septum; 

 ligamental pits linear, marginal, dorsal, anterior, with a white, nearly s[jongy margin within; muscu- 

 lar impressions superficial, the anterior small, rounded, the posterior large, subdorsal, uniform. 



Septifer hebertl Desh. is a characteristic species. 



41. Septifer bifurcatus Conrad. 



Mjii/us bifurcatus Conrad, Journ. Phil. Acad. Sci., Vol. VII, 1S37, p. 241, Fl. XVIII, fig. i^. 

 Rei£VE, Conch. Icon., PI. IX, fig. 41. 



Septifer bifurcatus Rve., Carpenter, Brit. Assn. Rept., 1863, p. 643. Gabb, Pal. Cal., Vol. II, 1869, 

 p. loi. Cooi'ER, 7th Ann. Rept. Cal. St. Min., 1888, p. 264. Keep, West Coast 

 Shells, [). 171, fig. 144, 1892. Williamson, Proc. U. S. Nat. Mus., Vol. XV, 1892, 

 p. 191. Dall, Trans. Wagner Inst. Sci., Vol. Ill, Part 4, 1898, p. 789. 



Shell small, wedge-shaped, equivalve, convex, rather thick; beaks pointed, terminal; surface 

 sculptured by numerous strong, rounded, terminally bifurcating, radiating ridges, and fine concentric, 

 incremental sulcations; margin corrugated; a small lamellar deck stretches across the interior of the 

 shell near the umbo; teeth small. 



Dimensions. — Beak to ventral margin 10 mm.; lat. 6 mm.; diam. 4 mm. 



This little shell is shaped like a MijliliD^ edulis, but is smaller and lias 

 [)roiiiinent, bifurcating, radiating ridges, delicate incremental sculpture, and an 

 nmbonal deck. 



Rare in the lower San Pedro series at Deadman Island. 



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



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



