NIAGARA GEOTJP. 



269 



628. 42. ATRYPA NITIDA, Var. OBLATA. 



PL. LV. Fig. 2 a, b. 



Shell subglobose, oblate, wider than long ; surface marked by fine concentric striae ; a broad 

 and undefined mesial depression towards the base of the dorsal valve. 



This species possesses many features in common with the preceding, though its greater size, 

 and want of connexion by specimens of intermediate size, induces a belief that it may be a 

 distinct species. 



Fig. 2 a. Ventral view of a specimen of this variety. 

 Fig. 2 6. -Dorsal view of a large specimen. 



Position and locality. In the shale at Lockport, and at Marshall's mill, Sweden, Monroe 

 county. 



629. 43. ATRYPA {Species undetermined). 



Pl. LV. Fig. 3a - c. 



Shell subglobose ; length a little greater than the width ; dorsal valve very gibbous, with a 

 depression from the centre to the base ; surface marked by concentric striffi and stronger lines 

 of growth ; beak of dorsal valve small, acute, and strongly incurved. 



This species is allied to the preceding variety of Jl. nitida, but presents a slightly different 

 form, a more defined and conspicuous sinus on the dorsal valve, which is more gibbous and has 

 a more acute and incurved beak.. The cast presents some peculiarities which I have not been 

 able to see in casts of ^. nitida. 



Fig. 3 a. Dorsal view of this species. 



Fig. 3 b. Front view of the same. 



Fig. 3 c. Cast of the dorsal valve of a specimen having the general external features of this 



species. 

 Fig. 3 d. Cardinal view of the same. 

 Fig. 3 e. Lateral view of the same. 



Position and locality. In the shale of the Niagara group at Lockport. 



630. 44. ATRYPA CRASSIROSTRA (n. s;;.). 



Pl. LV. Fig. 4 a, b, c. 



Shell elongated, subcylindrical, gibbous in the middle ; valves nearly equally convex ; beak 

 of the dorsal valve much elevated above the ventral valve, and abruptly incurved ; base of 

 dorsal valve extended into a linguiform process, filling a sinus in the base of the ventral valve ; 

 ventral valve having a broad, partially defined mesial elevation on the lower half of the shell ; 

 surface concentrically striated, with several stronger lines of growth. 



