ARNOLD— THE PALEONTOLOGY AND STItATIGRAPHY OF SAN PEDRO. 133 



Distiiijiiiisliable l)y small size, cancel latiul siirl'acc, and liiiiulc in Icl'l valve. 

 Specimens idontilied by Dr. Dall. 



Common in the lower San Pedro series of Deadiuan Island and San Pedro; 

 and in tlie upper San Pedro series of San Pedro, Los Cerritos, Crav^fisli George's, 

 Dcadman Island, and Long Beach. Found also in the Pliocene of Pacific F^cach, and 

 the Pleistocene of Twenty-sixth Street and Pacific Beach, San Diego. 



Living. — ^Monterey to San Diego (Cooper). 



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

 (Arnold). 



Pliocene. — San Diego (Arnold). 



62. Lucina tenuisculpta Carpenter. 



Lucina tenuisculpta Cpr., Brit. Assn. Re|:)t., 1S63, p. 642. Cooper, 7th Ann. Rept. Cal. St. Min., 

 18S8, p. 247. 



Shell small, orbicular, deeply convex, thin; umbones prominent, central ; surface sculptured 

 by numerous fine, concentric hnes and radiating strice; lunule prominent, the greater part being in 

 the right valve; cardinal teeth small, lateral.s prominent ; anterior muscle-impressions not as elongated 

 as in L. acutilineata and others. 



Dimensions. — Long. 9 mm.; all. 8.8 mm.; diam. 6.4 mm. 



Distinguishable by small size, sculptured surface, and great convexity. This 

 species resembles Diplodonta orhelUi very closely in shape and size, b;it may be 

 distinguished by the lack of [)rominent cardinal teeth, by its lateral teeth, and by the 

 radiating stria'. Specimens identified by Dr. Dall. 



Rare in the upper San Pedro series of San Pedro. Found also in the Pleis- 

 tocene of Spanish Bight, San Diego. 



Living. — Straits of Fuca to Catalina Island; Maztitlan (Cooper). 



Pleistocene. — San Pedro; San Diego (Arnold). 



Pliocene. — San Diego well (Dall). 



Family XXll. Dl PLOPONTl D.E. 



Genus Diplodonta Urovn. 



Section Diplodonta .s. .s. 



Shell rotund, equilateral, externally concentrically striated or smooth, with inconspicuous 

 epidermis; two cardinal teeth in each valve, of which the right posterior and the left anterior are 

 distally sulcate or bifid; no lateral teeth; the hinge-plate when developed is usually excavated distally; 

 there is no circumscribed lunule or escutcheon; the adductor scars are subequal, continuous with the 

 pallial line, and close to the hinge-plate; the margin is entire, the pallial line simple, and pallia! area 

 often radiately striate. 



Type, Dijflodonta Inpl.nv^ Brocchi. 



