Maryland Geological Survey 517 



differences are sufficiently obvious in Whitfield's two figures. Gabb's 

 species is much higher relatively, with higher, more prominent umbones, 

 a broader posterior keel, and a liner concentric sculpture. Leda whitfieldi 

 is quite common in the Monmouth of Prince George's County, Maryland. 



The form figured by Whitfield from Haddonfiekl, New Jersey, which 

 must serve as the type, is in the collection of the Philadelphia Academy of 

 Natural Sciences. 



Occurrence. — Monmouth Formation. Brightseat, Brooks estate near 

 Scat Pleasant, Friendly, 1 mile west of Friendly, McNeys Corners, Prince 

 George's County. 



Collections. — Maryland Geological Survey, Philadelphia Academy of 

 Natural Sciences. 



Outside Distribution. — Matawan Formation. Woodbury clay, New 

 Jersey. 



Leda rostratruncata n. sp. 

 Plate XIX. Figs. 8, 9 



Description. — Shell very thin and fragile, transversely elongated, 

 moderately convex, umbones small, not very prominent, incurved and 

 opisthogyrate, placed a little in front of the median horizontal; anterior 

 end a little shorter than the posterior, broadly rounded distally ; posterior 

 end strongly rostrate, the rostrum squarely truncated, and isolated by a 

 broad and shallow depression in front of it; lunule narrow but well 

 defined; escutcheon strongly differentiated, outlined by a low ridge, per- 

 sistent to the extremity of the keel; external surface sculptured with 

 thirty to forty fine, concentric ruga?, regular in size and spacing over the 

 entire disk but absent upon the tips of the umbones and the lunule and 

 escutcheon; teeth arranged in two discrete series, becoming finer toward 

 their point of convergence beneath the umbones; anterior series number- 

 ing approximately twenty-five denticles, the posterior twenty-eight; pal- 

 lial line and muscle scars obscure. 



Dimensions. — Eight valves, longitude 18..5 mm., 8.5 mm.; left valve 

 of another individual, longitude 14.5 mm., altitude 7 mm. 



