NIAGARA GROtnP. 



27& 



640. 54. ATRYPA OBTUSIPLICATA («. 5^.). 



Pl. LVIII. Fig. 2a .h. 



Spheroidal, or more or less gibbous ; ventral valve extremely convex ; beak of dorsal valve 

 small, closely incurved over the ventral valve ; surface plicated ; plaits simple, rounded, about 

 18 to 20, three or four of which are depressed in the dorsal valve and projecting in front, 

 filling a deep sinus in the margin of the ventral valve ; a corresponding elevation on the ventral 

 valve, which reaches from the base two-thirds of the way to the beak ; plications crossed by 

 fine subimbricating concentric strise. 



This shell is easily distinguished from the other species of this group, by its rounded form 

 and obtuse plications in the perfect shell, which are subangular in the cast. The proportion of 

 the two valves is variable, the ventral one often becoming extremely convex, with the sinus 

 in front greatly elevated. The number of plications varies from sixteen to twenty-two in the 

 greatest extremes of size I have seen ; while three, and rarely four, are depressed on the one 

 valve, and four or five elevated on the opposite valve. 



Fig. 2 a, h, c. Ventral, dorsal and profile views of a specimen of the ordinary size. 



Fig. 2 d. Front view of the same. 



Fig. 2 e. Ventral view of a specimen which is more expanded at the sides. 



Fig. 2 f. Front view of a specimen where the ventral valve is much elevated, and the sinus 



verj' deep. 

 Fig. 2 g. Ventral valve of a compressed specimen. 

 Fig. 2 k. Several plications enlarged, showing the concentric strire. 



Position and locality. In the shale and lower part of the limestone at Lockport. 



641. 55. ATRYPA PLICATELLA? 



PL. LVIII. Fig. 3a-c; and4a, b. 



Terebratula (^jinomites) plicatella, Linne, Wahi^enberg, Dalsian, Hisinger, &c. 

 Terebratula lacunosa, Schlotheim, Nachtr. 1, pag. 68, pl. xx, fig. 6. 



— — SowERBY ; MuRCHisoN, SU. System, pag. 624, pl. 12, fig. 10. 



Not T. lacunosa, Dalmaiv and Hisinger. 



Obovate, outline subtriangular ; surface strongly plicated ; plications simple, acute, crossed 

 by prominent threadlike stria; ; beak of the dorsal valve sharp, slightly extended, and scarcely 

 incurved ; three or four plications on the dorsal valve depressed, and strongly elevated in 

 front ; an equal number of plications on each .'~ide of the mesial sinus and elevation. 



This species differs from the preceding in the more angular form of the shell, the extended 

 sharp beak of the dorsal valve, and in the sharp prominent striae, which contrast in a very de- 

 cided manner with the rounded striae and closely incurved beak of A. obtusiplicata. The figure 

 of T. lacunosa (SowEBBy), from the Wenlock limestone, bears a close resemblance to our fossil, 



