CLINTON GEOUr. 



77 



presslon extending from the base nearly to the curve of the beak ; lateral margins of the two 

 valves uniting in nearly a direct line ; surlace marked by fine concentric lines, and stronger 

 lines of grow^th ; towards the margin, some fine radiating strite are visible. 



The peculiar form of this species distinguishes it from all the others of the group. It is nearly 

 cylindrical, the depth of both valves, where the shell is closed, being equal to the width. 

 The beak of the dorsal valve is much elevated above the ventral valve, but is not angulated 

 as in the preceding species. There is a slight sinuosity in the front of the ventral valve, filled 

 by a corresponding projection of the dorsal valve ; but in ordinary specimens, this feature is 

 scarcely observable. All the specimens seen are more or less exfoliated, so that the entire 

 character of the surface can not be distinctly seen. The longitudinal striae are quite effaced on 

 some specimens, while in others they are very distinct towards the margin. 



Fig. 2 a. View of the ventral valve, showing the beak of the dorsal valve elevated above it. 



Fig. 2 h. Dorsal valve of the same. 



Fig. 2 c. Profile view of the same. 



Fig. 2 d, e^ f. Ventral, dorsal and profile views of a smaller shell. 



Fig. 2 g Cardinal view of the last. 



Fig. 2 h. Front view of the same. 



Position and locality. In the upper limestone of the group at Lockport, Niagara county. 

 Probably also in other places in the same position. (Collection of Col. Jewett.) 



466. 37. ATRYPA INTERMEDIA. 



Pl. XXIV. Fig. 3 a ~f, and 4a-d. 



Shell obovate, rapidly expanding in width from the beak nearly to the base, which is ab- 

 ruptly rounded ; length and width nearly equal ; valves almost equally convex, gibbous near 

 the beaks, but becoming flattened below ; front margin slightly sinuous ; surface marked by 

 concentric striae and a few lines of growth. 



This species, at first view, appears like one of the preceding ones flattened by pressure ; but 

 after examining a large number, I am satisfied of its specific distinction. It occurs likewise 

 with ^. cylindrica, and has been subjected to the same conditions. It is readily known by 

 being much compressed towards the base, the beak lesS incurved than in the preceding forms, 

 and its great width, which is nearly or quite equal to the length. The cast of this species is 

 likewise peculiar, and serves to show that it is quite distinct from either of the preceding. 



Fig. 3 a. View of the ventral valve. 



Fig. 3 b. Profile view of the same. 



Fig. 3 c. Cardinal view. 



Fig. 3 d. Front view, showing the scarcely sinuous margins. 



Fig. 3 e. A specimen with the shell partially removed, showing three strong elevated lines in the 

 cast. These elevated lines are laminae, marking the centre and margins of the mus- 

 cular impression, but in entire casts they are not conspicuous. 



