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. 8. 

 PI. 29, Fig. 241. 



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

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

 gon, near the stage road. The heaks 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, 11. 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. elliptica, nobis, from New Jersey, by 

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

 absence of any ridge. 



