ARNOLD — THE PALEONTOLOGY AND STKATIGKAPHY OF SAN PEDRO. 119 



Shell of medium size, wedge-shaped, rounded behind, thin; surface smooth, except for 

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

 teeth minute. 



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



The only difference between the Pleistocene specimens and the living form 

 is in the coloraiion of the epidermis, that of the former being reddish brown, while 

 the latter is blue. Specimens identified by Dr. Dall. 



Rare in the lower San Pedro series of Deadman Island; and in the upper 

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



Living. — San Pedro (Williamson): Monterey, north; Japan; circiimpolar 

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



Pleistocene. — Benicia, Solano County (Cooper): San Pedro (Arnold): Atlantic 

 coast from Labrador to St. John's River, Florida; northern Europe; northwest coast 

 of Ameriqa (Dall). 



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



Genus Septifer Reduz. 



Shell equivalve, very inequilateral; ventral margin 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 spongy margin within; muscu- 

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



Septifer heberti Desli. is a characteristic species. 



41. Septifer bifurcatus Conrad. 



Mylilus bifutcaliis Conrad, Journ. Phil. Acad. Sci., Vol. VII, 1837, p. 241, PI. XVIII, fig. 14. 

 Reeve, Conch. Icon., PL IX, fig. 41. 



Septi/cr bifurcatus RvE., Cakpenter, Brit. Assn. Rept., 1863, p. 643. Gabb, Pal. Cal, Vol. II, 1869, 

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

 Shells, p. 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 Mylilvs edulis, but is smaller and has 

 prominent, bifurcating, radiating ridges, delicate incremental sculpture, and an 

 umbonal deck. 



Rare in the lower San Pedro series at Deadman Island. 



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



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



