134 CALIFORNIA ACADEMY OF SCIENCES. 



63. Diplodonta orbella Gould. 



Plate XVIII, Figs. 8 and 8rt. 



Lticina orbella Gld. , Proc. Bost. Soc. Nat. Hist., Vol. IV, 1851, p. 90; Bost. Jour. Nat. Hist., Vol. 



VI, 1853, p. 395, PI. XV, fig. 3; Cal. & Mex. Shells, p. 22, PI. XV, fig. 3. 

 Diplodonta orbella Gld., Gould & Carpenter, Proc. Zool. Soc, 1856, pp. 202, 218. '=( Mysia) 



Sphairella tumida CoN. {Jide Cpr., Brit. Assn. Rept. , 1863, p. 643). Cooper, 7th Ann. 



Rept. Cal. St. Min., 1888, p. 238. Keep, West Coast Shells, p. 179, 1892. Williamson, 



Proc. U. S. Nat. Mus., Vol. XV, 1892, p. 190. Dall, Trans. Wagner Inst. Sci., Vol. 



Ill, Part 5, 1900, p. 1189. 



Shell small, subglobose, thin; beaks nearly median, not prominent; surface sculptured with 

 fine incremental lines, which at some parts are more conspicuous than at others and render the surface 

 somewhat irregular; no distinct lunule; ligament prominent; two cardinal teeth in right valve, of 

 which the anterior is smallest, and the posterior bifid; and two in the left valve, of which the anterior 

 is bifid, and the posterior very oblique; no lateral teeth; muscle-impressions large, faint. 



Dimensions. — Long. 11. 9 mm.; alt. 11. 9 mm.; diam. 9 mm. 



Kesembles Lucina tenuiscidpia, but may be distinguished by lack of radiating 

 stria?, and by the prominent cardinal teeth and hick of lateral teeth. Sjiecimens 

 identified by Dr. Dall. 



Rare in the lower San Pedro series at Deadman Island; and the upper San 

 Pedro series of San Pedro and Crawfish George's. Found also in the Pleistocene of 

 Twenty-sixth Street, San Diego. The specimen figured is from the lower San Pedro 

 series of Deadman Island, and is now in the collection of Delos Arnold. 



Living. — Straits of Fuca to San Diego (Cooper). 



Pleistocene. — San Pedro to San Diego (Cooper): San Pedro; San Diego 

 (Arnold). 



Pliocene. — San Diego well (Dall). 



64. Diplodonta serricata Reeve. 



Plate XVIII, Fios. 5 and 5n. 



Diplodonta serricata Rye., Cpr., Brit. Assn. Rept., 1S56, p. 248. 



Shell small, orbicular, only slightly convex, thin; umbones central, not prominent ; surface 

 sculptured with numerous fine, incremental lines, which are regular in prominence; lunule not distinct; 

 hinge prominent; teeth and interior as in D. orbella. 



Dimensions. — Long. 13.8 mm.; alt. 14 mm.; diam. 7 mm. 



Distinguishable from D. orhclla by its much less convexity and more regular 

 concentric lines. Specimens identified by Dr. Dall. 



Not uncommon in the upi^er San Pedro series of San Pedro. This species is 

 very common in the Pleistocene deposits at Twenty-sixth Street, San Diego. The 

 specimen figured is from this horizon, and is now in the collection of Delos Arnold. 



Living. — Mazatlan (Carpenter). 



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



