AKNOLD — THE I'ALEONTOLOC.Y AND Sl'UATU'.UAril V OF SAN PEDRO. 11'.) 



Shell of medium size, wedge-shaped, rounded behind, thin; suilace 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 s|Kciineris and the living t'orm 

 is in the coloration of the e}>idetniis, lliat of the former being leddish brown, while 

 tiie latter is bhie. Specimens itieiitifieil by Dr. Dall. 



Rare in the lower San Petiro series of Deadman Island; ami in the nppei' 

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



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

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



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

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

 of America (Dall). 



Pliocene. — Red Crag, (Jreat 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; mu.scu- 

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



Septifer heberti Desh. is a characteristic species. 



41. Septifer bifurcatus Conrad. 



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

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



5f/>///,»r^///<;'ra/«i Rye., Carpenter, 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, conve.x-, 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.— ^^?i& to ventral margin 10 mm.; lat. 6 mm ; diam. 4 mm. 



This little shell is shaped like a Mijldm^ edu/iii, 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). 



