CRETACEOUS FOSSILS. 199 



Figures 184, and 184 a, natural size ; 184 b, magnified, to show the ribs. 



Localities: Division A., at Pence's Ranch, Butte County; Tuscan Springs, 

 Tehama County ; Ranch of San Luis Gonzaga, east end of Pacheco's Pass ; and 

 in Division B., not rare, at Martinez. 



Closely allied to N. divaricata, Con., Wilkes's Expedition, Geology, pi. 18, fig. 

 6; but differs in the abrupt anterior truncation, the front of Conrad's species 

 being sinuous, and the lunular depression absent. 



LEDA, Schum. 

 L. protexta? Gabb. 



PL 20, Fig. 185. 

 (Id. Jour. Acad. Nat. Sciences, Phila., 2 ser., vol. 4, p. 303, pi. 48, fig. 23.) 



Shell small, elongated, narrow; anterior end produced, 

 rounded; posterior end long, narrow, curved upwards; base 

 broadly and pretty regularly convex; beaks small, subcentral, 

 not prominent, with the cardinal margin somewhat convex in 

 front, excavated behind. Surface marked by numerous, regular, 

 concentric ribs. 



Figure, somewhat magnified. 



Localities: Common in the strata overlying the coal, from Clayton to Marsh's, 

 also found at Martinez ; San Diego ; Alizos Creek, near Canada de las Uvas, and 

 in the San Emidio Canon, near the same place (Division B.). It also occurs in 

 Division A., near Martinez. 



This shell resembles so closely the species described under the above name from 

 New Jersey and Tennessee, that I have not ventured to separate it. A careful 

 comparison of specimens from the two sides of the continent might bring to light 

 differences of specific value. 



L. TRANSLUCIDA, U. S. 

 PL 30, Fig. 200. 



Shell minute, inequilateral, convex; beaks about two-fifths 

 from the anterior end, which is produced and acutely rounded; 

 base regularly convex; posterior side produced, acute below, 

 forming nearly a straight line from the angle to the beak; this 



