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PALjEONTOLOGY of NEW-YORK. 



shell, however, the beak is never extended and perforate at the apex, as in the species cited, 

 nor is the mesial sinus and ridge so broad ; the surface is marked by fewer plications, which 

 are always crossed by strong, concentric lamelte. 



In the young shells of this species the ventral valve is nearly flat, as shown in the figure, and 

 there is no distinct ridge upon the dorsal valve. In the next stage we find a distinct sinus and 

 elevation, which are more conspicuous as the shell becomes larger. The plications number 

 from four or five to nine on each side of the central lobe and sinus ; and they are crossed by 

 prominent imbricating lamellae, which, when worn off, leave the plications nodulose. This 

 shell may readily be mistaken for the following, but a comparison of the two with the descrip- 

 tion will enable any one to distinguish them. 



Fig. 1 a. The dorsal valve of a young individual, which is marked by two stronger plications in 



the centre. 

 Fig. 1 b. Cardinal view of the same, showing the nearly flat ventral valve. 

 Fig. 1 c. Ventral view of a larger specimen, where the mesial lobe and sinus are developed. 

 Fig. 1 d. Profile view of the same specimen, showing convexity of both valves. 

 Fig. 1 e. Front view of the same. 



Fig. 1 /. Dorsal valve of another specimen, where the sinus is less developed. 

 Fig. 1 g. Front view, showing the slight mesial depression and elevation. 

 Fig. 1 h, i. Dorsal and ventral valves of a specimen of medium size. 

 Fig. 1 k. Front view of the same, showing the strong elevation in the centre. 

 Fig. 1 I. Profile view of the same. 

 Fig. 1 m. Dorsal valve of a large individual. 

 Fig. 1 n. An enlargement of the plications, showing character of surface. ' 



Position and locality. In the shale of the Niagara group at Lockport. 



632. 46. ATRYPA NODOSTRIATA (n. .?;).). 



Pl. Ln. Fig. 2o-w. 

 Compare Terebratula imbricata, etc., as above. 



Shell subrhomboidal, depressed globose ; beak of dorsal valve small, short, and slightly 

 elevated ; valves nearly equal, or the ventral valve deeper than the dorsal valve ; surface 

 marked by strong rounded plications, which bifurcate several times before reaching the margin ; 

 plications crossed by strong imbricating laminae, which, where worn, give a nodulose character 

 to the plications; dorsal valve in old specimens with a broad undefined mesial depression, 

 which extends half way to the beak ; a corresponding elevation on the ventral valve, which 

 does not assume the character of a defined ridge. 



This species is very similar to the preceding, but differs in all its stages of growth. In the 

 young shells, the valves are nearly equally convex ; in the more advanced condition, there is 

 scarcely any evidence of a mesial sinus or depression, but this feature gradually becomes con- 

 spicuous, and finally there is a broad depression, without defined margins in the dorsal valve, 



