NIAGARA GROUP. 



271 



different aspects which it presents in the different rocks, and in rocks of different age, though 

 similar or alike in their lithological and mineral character. This is fully exemplified in the 

 contrast between specimens from the shale of the Niagara group and the shale of the Hamilton 

 group, which are very similar rocks. 



;: In the present instance I have presented the principal varieties of aspect under which this 

 fossil appears in the shale of the Niagara group, from the young to the full grown individuals. 

 The imbricating lamellse of the shell in this geological position are not so prominent as in some 

 of those in a higher position, while they are nevertheless quite conspicuous. 



Fig. 5 a. A young individual of this species. 



Fig. 5 b. Cardinal view, shotring the compressed form. 



Fig. 5 c. A larger individual. 



Fig. 5 d, e. Cardinal and profile views of the same. 



Fig. 5 /, g, h. Ventral, profile and front view of a specimen, showing, in the last figure, the 



slight sinus in the dorsal valve, and the elevation of the margin of the ventral valve. 

 Fig. 5 i Profile view of a flattened specimen. 



Fig. 5 k, I, VI. Ventral, profile, and cardinal view of a globose specimen of medium size. 

 Fig. 5 71, 0. Dorsal and front view, showing the sinus more conspicuously than the preceding 



figure. 

 Fig. 5 p. A large individual : view of the ventral valve. 

 Fig. 5 r. Profile view of the same specimen, which is compressed. 

 Fig. 5 s. The interior of the dorsal valve, showing the form of the muscular impression. 



The foramen is not very distinctly defined, but there is no perforation of the beak, 



though in other specimens there is a minute perforation in the beak of the dorsal 



valve. 

 Fig. 5 t. Several of the plications enlarged, showing more distinctly the character of the surface. 



Position and locality. In the shale of the Niagara group, and also in the lower part of the 

 succeeding limestone. It is extremely abundant at Lockport, though the specimens are usually 

 compressed. It is likewise common in all the localities of this rock, from Wolcott in Wayne 

 county, to the Niagara river. 



631. 45. ATRYPA RUGOSA (n. 6/).). 



PL. LVI. Fig. la-re. 



Rhoraboidal, circular or suboval ; valves almost equally convex ; beak of the dorsal valve 

 elevated above the ventral valve, and abruptly incurved ; surface marked by strong plications, 

 which bifurcate once, twice or thrice before reaching the margin ; dorsal valve with a strong 

 mesial depression, in the bottom of which are one or two plications ; a corresponding ridge 

 on the ventral valve, with two or three plications ; plications crossed by strong elevated 

 lamellse, which give a very rugose character to perfect specimens. 



This species is closely allied to the following, and both possess many characters in common 

 with Terebratula imbricata of Murchison (Sil. System), T. marginalis of Dalman. In our 



