NIAGARA GROUP. 



277 



distinction. Few perfect specimens have been found, notwithstanding it is a very common 

 species : they are usually much depressed and distorted, and it is only after some experience 

 that one learns to recognize the species in its various phases. The Swedish specimens of this 

 species, with which I have had an opportunity of comparing our own, are usually better pre- 

 served, and the proportions less distorted. In other respects, there is no difference between 

 the specimens from these distant localities. 



Fig. 4 a, I. Ventral views, of two specimens of this species. 



Fig. 4 c, d. Dorsal views of a specimen of ordinary size, and of another of very large dimensions. 



Fig. 4 e. Dorsal view of a specimen having a greater proportional width than is usual. 



Fig. 4 /, g, h. Profile views of specimens of different size. 



Fig. 4 i. Profile of fig. 4 e, showing the great depth of the ventral valve. 



Fig. 4 yt - p. Front views of young and old specimens, showing the former without a sinus 



and its gradual increase to 4 p, which is a front view of the specimen shown in fig. 



4 e and i. 

 Fig. 4 r. Several plications enlarged, showing the concentric filiform striae. 



Position and locality. In the shale at Lockport, and on the Niagara river above Lewiston : 

 more rarely in other localities of the same rock. 



637. 51. ATRYPA 1 



PL. LVIl. Fig. 5 a, h. 



Several specimens, having the character of the one figured, have been seen. The shell is 

 cuneate and compressed, the base rounded. Along the centre of the ventral valve there is a 

 depressed line, from which the plications diverge. This feature may arise from a sharp de- 

 pression along the centre, which has infolded the plications, giving them the appearance of 

 radiating or diverging from this line ; though it is scarcely possible that such an accident could 

 have produced the characters exhibited. 



Fig. 5 a. Ventral view. 



Fig. 5 b. Dorsal view, showing the simple direct plications. 



Position and locality In the shale at Lockport, associated with the preceding species. 

 638. 52. ATRYPA DISPARILIS ( 71. 5^.). 



Pl. LVir. Fig.6a~m. 



Suboval, plano-convex ; dorsal valve very convex, arcuate, with the beak extended and 

 perforate at the apex, slightly curved over the ventral valve, which is flat or concave ; surface 

 strongly plicated, the dorsal valve with two very prominent plications down the centre, and two 

 three or four smaller ones on each side ; ventral valve with a smaller plication down the 

 depressed centre, and two to four on each side ; plications crossed by fine, nearly obsolete 

 concentric strige, and a few stronger imbricating lines of growth. 



