NIAGARA GROUP. 



283 



Fig. 1 a. A young individual of this species, having the radiating and concentric striae equally- 

 developed. 



Fig. 1 b. A larger individual : this one is about the ordinary size of the specimens obtained. 



Fig. 1 c. A large individual, preserving the surface markings in a good degree of perfection. 



Fig, 1 d. The right valve of this species, showing the plain nearly flat body of the shell, and 

 striated wing. 



Fig. 1 e. An enlargement of a portion of the surface, showing the character of the diverging and 

 concentric striae. 



Position and locality. In the shale of the group at Lockport, Rochester, and other places. 



644. 8. AVICIJLA UNDATA (n. sp.). 



PL. LIX. Fig. 2. 



Subrhomboidal, oblique ; left valve convex, much elevated in the middle, umbo prominent ; 

 surface marked by strong concentric striae and stronger undulations ; posterior wing distinct, 

 extending nearly or quite as far as the posterior margin of the shell ; anterior wing short, 

 rounded at the extremity. 



The form of this shell is very similar to the last, except that it is somewhat less oblique to 

 the cardinal line. The surface is strongly marked by concentric lines, without radiating strise. 



Position and locality. In the shale of this group at Rochester. 



645. 9. AVICULA SUBPLANA (?i. sjt).). 



Pl. LIX. Fig. 3 a, b, c. 



Extremely depressed ; left valve subrhomboidal the height equal to about three-fourths of 

 the length ; elevated towards the umbo, and nearly flat below the centre of the valve ; posterior 

 wing scarcely distinct from the body of the shell, truncated at its extremity ; cardinal line 

 equal to or less than the length of the shell ; surface marked by concentric strige, which are 

 scarcely undulated on the wing ; right valve smaller, nearly flat, with the wing more extended, 

 surface similarly marked ; anterior wing on both valves scarcely conspicuous. 



The general appearance of this shell is similar to the right valve of ^. emacerata ; but it is 

 distinguished by the similarity of the two valves where both are present, and, in either one, 

 from the less distinctness of the wing, the absence of radiating striae on the body or the wing, 

 and of undulations in the concentric striae as they pass from the body of the shell to the wing. 

 A careful examination shows faint radiating undulations upon the surface of both valves, but 

 they never become prominent striae. 



This shell is associated with A. emacerata.^ but is a less common species. It approaches in 

 many respects the A. naviformis of Conrad {A. retrojlexa of Hisinger), but is less convex 

 and the surface less conspicuously striated ; but occurring only in the soft shale, the surface 

 markings are probably not so prominent as they would be under other circumstances. 



