94 STUDIES IN GEOLOGY, No. 3 



dorsal margin about straight. Umbones pointed, fairly 

 prominent, opisthogyrate, central in position. Surface gen- 

 erally convex except behind the posterior keel, where a well- 

 defined area is concave on either side of the juncture of the 

 valves. The keel is sharp, and broadly curved, the concave 

 side upward. Sculpture of concentric, sharp and clearly 

 graven ribs, asymmetric in cross-section, the dorsal side 

 steeper, about 14 in the space of 5 mm. on the median surface 

 of a specimen 19.5 mm. long. On the posterior area the 

 ribs are radial, running nearly parallel to the juncture of the 

 valves. Teeth normal, fairly strong in the center of each 

 series, and becoming smaller distally and toward the beaks, 

 A-shaped, the apices turned toward the beaks. Inner shell 

 smooth. There is no well-defined lunule. Dimensions of 

 two specimens: Length, 19.5; alt., n; diam., 8.5 mm. 

 Length, 25; alt., 15; diam., 14 mm. 



This form was named Leda acuminata by Nelson (loc. 

 cit.), but, that name being preoccupied, Dall proposed Leda 

 peruviana in 1898. It is identical with the subsequently 

 named Leda balboae Brown and Pilsbry (loc. cit.) from 

 Gatun. Specimens in Nelson's collection agree perfectly with 

 the figure of balboae published by Brown and Pilsbry, and 

 the points noted in their description tally perfectly, excep- 

 ting the number of teeth, which is given as 18 behind the 

 beak and 28 before for the Gatun specimens. The Zorritos 

 forms have constantly 14 teeth behind the beak, and 22 

 before. Other points, however, agree to> such fineness of 

 detail that the two can hardly be held to be of different 

 species. 



Zorritos formation. Zorritos. 



