NIAGARA GROUP. 283 
Fig. 1 s. A young individual of this species, having the radiating and concentric strise 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 strise. 
Position and locality. In the shale of the group at Lockport, Rochester, and other places. 
644. 8. AVICULA UNDATA (n. sp .). 
Pl. LIX. Fig. 2. 
Subrhomboidal, oblique ; left valve convex, much elevated in the middle, umbo prominent; 
surface marked by strong concentric strise 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. AYICULA SURPLANA (n. sp.). 
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 strise, 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 A. 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 strise 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. retroflexa 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. 
