ARNOLD — THE PALEONTOLOGY AND STKATIGUAPHY OF SAN PEDUO. 95 



MOLLUSCA. 



Class PELECYPODA." 



Order I'RIOXODESMACEA. 



Superfamily NUCULACEA. 



Family VI. NUCULIDiE. 



Genus Nucula Lamarck. 



Slu-U trigonal, with llic umhones 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 //. £ A. Admns. 

 Shell with divaricate sculpture. 

 Nucula divaricata Hinds k a characteristic species. 



II. Nucula (Acila) castrensis Hinds. 



Nuaila castrensis Hinds, Proc. Zool. Soc, 1843, P- 98; Zool. Sulph., p. 61, PL 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, Jide Carpenter, Brit. Assn. Rept. , 1863, p. 644); (=Nucu/a 



divaricata CoN.=N. decisa CoN.==N. conradi Meek, fide 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, i8g8, 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 (listingiiisliable by the hinge teeth and divaricate scidpture. Speci- 

 mens identified by Dr. DalL 



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

 San Pedro. 



LiciiKj. — Sitka to San Diego (Cooper). 



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

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

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

 Blakcly, near Seattle, Washington (Arnold). 



1 Tbe major clossllicatjou of tbls class is accoMlng to that preeented by Dr. W. H. Dall in Part Ut of the " CoDtribiitious to the 

 Tertiary Fauua of Florida." Tbo minor (let.iUs of claMilfication and most of tbe generic descriptions are from Parts IV and V of tlie same 

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



