NIAGARA GROUP. 273 



and a corresponding elevation in the ventral valve. It never presents the decided mesial sinus 

 and lobe of A. rugosa. The plications, of which there are from ten to twenty on each valve, 

 are more rounded than those of that species, though this character is not sufficient for discrimi- 

 nation. The shell is larger, often very rotund in its perfect condition, and, when compressed 

 or distorted, may easily be mistaken for Ji. reticularis with which it is associated. 



From the resemblance of this fossil to the figures of Murchison, PL 12, fig. 12, I had 

 been inclined to refer it to the same species ; but a more careful comparison, together with a 

 direct comparison with Swedish and Bohemian specimens of T. marginalis (Dalman), has 

 led to a different conclusion. In our specimen the beak is never straight and extended, but 

 small and closely incurved : there is sometimes a perforation, which appears more as if due to 

 accident than nature ; but this does not correspond with the perforation in T. imbricata {T. 

 marginalis). The absence of a defined mesial sinus in our specimens is another feature of im- 

 portance, as well as the coarser plications with their strong imbricating lamellee. 



In its largest dimensions our species exceeds the foreign analogue, approaching in size to 

 the A. reticularis of this period, from which it can be readily distinguished by its stronger 

 plications. 



Fig. 2 a, b. Dorsal and ventral view of a young specimen. 



Fig. 2 c. Profile view of the same specimen, which is compressed. 



Fig. 2 d, e. Dorsal and ventral view of a specimen below the medium size. 



Fig. 2 /. Profile view of the same. 



Fig. 2 g, h. Dorsal and ventral views of a specimen of medium size, which is slightly more 



elongated than usual. 

 Fig. 2 ?, k, I. Dorsal, ventral and profile view of a more rotund form. 

 Fig. 2 VI, n. Ventral and profile view of a large specimen in its natural condition. 

 Fig. 2 0. Profile view of a larger specimen which is extremely compressed. 

 Fig. 2 p. The plications enlarged, showing the imbricating lamellje. 

 Fig. 2 r, 5, t, u. Front views of different specimens, showing the absence of a sinus in the 



young shell, and its increase as the shell becomes larger. 



Position and locality. In the shale at Lockport. 



633. 47. ATRYPA CAMURA (n. sp.). 



Pl. LVI. Fig. 3a-t. 

 Subrhomboidal, semioval or depressed globose (varying in form by age) ; valves nearly 

 equally convex, or the ventral a little deeper than the dorsal ; beak of the dorsal valve small, 

 acute, projecting beyond the cardinal line of the ventral valve, and slightly incurved ; surface 

 marked by simple angular plications on each side, and by one or two smaller ones in the 

 centre ; plications crossed by fine threadlike concentric striae, with a few stronger imbricating 

 lamellae below the centre ; an oval aperture or foramen below the beak. 



[ PAiJEONTOLOBT — VoL. II.] 35 



