CRETACEOUS FOSSILS. 151 



Figure, natural size. 



Locality: Two specimens, collected by Mr. Mathewson, at Martinez. 



Named after my friend, Mr. George W. Tryon, of Philadelphia. 



A. INEQUILATERALIS, 11. S. 

 PI. 29, Fig. 241. 



I propose this name for an imperfect cast, which I discovered 

 in Division A., on the north side of the Siskiyou Mountains, Ore- 

 gon, near the stage road. The beaks are about a third of the 

 length from the posterior end; the cardinal and basal margins are 

 subparallel; and, judging from undulations on the cast, the two 

 ends of the shell are nearly of the same shape and size. Surface 

 unknown. 



Figure, natural size. 

 Collection of Prof. Whitney. 



? A. LATA, n. s. 

 PI. 22, Fig. 126. 



Shell broad, thin, lenticular; valves very much flattened; 

 beaks small, inclined backwards, placed posterior to the middle 

 of the shell. Anterior cardinal margin sloping convexly from 

 the beak; posterior excavated. Posterior end truncated. Sur- 

 face marked by minute, irregular lines of growth. 



Figure, natural size. 



Locality: A single specimen from Pence's Ranch, Butte County (Division A.). 



I have not been able to expose the hinge of this 'specimen ; but, judging from 

 the general form, think there can be little doubt of the correctness of the generic 

 reference. It can be distinguished from A. ellij>tka, nobis, from New Jersey, by 

 its being shorter and broader, by the beak being more posterior, and by the entire 

 absence, of any ridge. 



