ARNOLD — THE PALEONTOLOGY AND STKATIGKAPHY OF SAN PEDRO. 95 



MOLLUSC A. 

 Class PELECYPODA.' 



OnUn- I'RIONODESMACEA. 



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 carlilage- 

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



Tj'pe, Area nucleus Lara. 



Subgenus Acila //. <i- A. Adams. 

 Shell with divaricate sculpture. 

 Nucula divaricata Hiiuls is a characteristic species. 



n. Nucula (Acila) castrensis Hinds. 



Nuaila castrensis Hinds, Proc. Zool. Soc, 1843, P- 9^1 Zool. Sulph., j). 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,7?</<f Carpenter, Brit. Assn. Rept., 1863, p. 644); {=^Nucula 



divaricata CoN.=N. decisa CoN.=N. conradi Mekk, /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, conve.K, of medium thickness; umbones posterior to center, turned 

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

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

 each side. 



Dimensions. — Long. 11.5 mm.; alt. 10 mm.; diam. 8 mm. 



Easily distingui.sliable by the liinge teeth and divaricate sculpture. Speci- 

 mens identified by Dr. Dall. 



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

 San Pedro. 



Living. — 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 classificatiou of thig claBs is according lo that presented by Dr. W. H. DaU in Part III of the " Contributions to tbo 

 Tertiarj' Fauna of Florida." The minor details of classiftcation and most of tbe generic descriptions are from Parts IV and V of tbe same 

 work. Tbe rest of tbe generic descriptions are from Trj-on's '• Structural and Systematic Conchology." 



