AKNOLD — THE PALEONTOLOGY AND STRATIGBAPHY OF SAN PEDRO. 95 



MOLLUSCA. 



Class PELECYPODA.' 



Order PRIONODESMACEA. 



Superfamily NUCULACEA. 



Family VI. NUCULID.E. 



Genus Nucula Lamarck. 



Shell trigonal, with the umbones turned towards the short posterior side; smooth or sculp- 

 tured; epidermis olive; interior pearly; margins crenulated; hinge with prominent internal cartilage- 

 pit, and a series of sharp teeth on each side; pallial line simple. 



Type, Area nucleus Lam. 



Subgenus Acila H. & A. Adams. 

 Shell with divaricate sculpture. 

 Niicula divaricaia Hinds is a characteristic species. 



II. Nucula (Acila) castrensis Hinds. 



Niicula castrensis Hinds, Proc. Zool. Soc. , 1843, p. 98; Zool. Sulph., p. 61, PI. XVII, fig. 5, 1844. 



H. &. A. Adams, Gen. Rec. Moll., Vol. II, p. 545. Williamson, Proc. U. S. Nat. Mus., 



Vol. XV, 1892, p. 192. 

 Acila castrensis Hinds {^=A. lyalli BAiRD,yide Carpenter, Brit. Assn. Rept., 1863, p. 644); {^Nucula 



divaricata CoN.^JV. decisa CoN.=N. co/iradi Meek, /ide Gabb, Pal. Cal., Vol. II, 1869, P" 



102). Cooper, 7th Ann. Rept. Cal. St. Min., 1888, p. 227. Williamson, Proc. U.S. Nat. 



Mus., Vol. XV, 1892, p. 192. Dall, Trans. Wagner Inst. Sci., Vol. Ill, 1898, p. 572. 



Shell small, trigonal, convex, of medium thickness; umbones posterior to center, turned 

 posteriorly; anterior end longer than posterior, rounded; posterior end short, truncated; surface 

 divaricately sculptured; hinge with prominent internal cartilage- pit and numerous sharp teeth on 

 each side. 



Dimensions. — Long. 11. 5 mm.; alt. 10 mm.; diam. 8 mm. 



Easily distinguishable by the liinge teeth and divaricate sculpture. Speci- 

 mens identified by Dr. Dall. 



Common in Pliocene, rare in lower San Pedro series, of Deadman Island and 

 San Pedro. 



Livincj. — Sitka to San Diego (Cooper). 



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

 Pliocene. — San Pedro (Arnold) : San Fernando; San Diego well (Cooper). 

 (?) Miocene. — Oregon; Martinez; Griswold's, San Benito County (Cooper): 

 Blakely, near Seattle, Washington (Arnold). 



1 The major classiticatiou of this class is according to that presented by Dr. W. H. Dall iu Part III of the " Contributions to the 

 Tertiary Fauna of Florida." The minor details of classificatiou and most of the generic descriptions are from Parts IV and V of the same 

 work. The rest of the generic descriptions are from Tryon's " Structural and Systematic Conchoiogy." 



