CLINTON Q-aoXTP. 



71 



Fig. 5 a, b. Ventral and profile view of a small specimen. 



Fig. 5 c, d, e. Dorsal valves of three individuals of different size. 



Position and locality. In the arenaceous layers near the base of the grouji in Oneida county, 

 Blackstone's quarries. (niate Collection.) 



458. 29. ATRYPA EMACERATA. 



Pl. XXIII. Fig. 6 a, b, c, d. 



Flabelliform, subrhomboidal, trilobate ; surface plicated ; plications about eighteen, six of 

 them raised in front on the mesial lobe, usually bifurcating equally, those on the mesial lobe 

 often having a smaller one intercalated ; concentrically marked by undulating imbricating 

 lamellee. 



This species is known only in the green shales of Sodus and Rochester. All the specimens 

 seen are flattened, and the shell partially or entirely destroyed. In the specimens figured, 

 although the ribs are distinctly preserved over the entire surface, the outline and extent of the 

 muscular impression are clearly perceptible. On the sides of the shell, the ribs are distinctly 

 and equally bifurcating above the centre, and often near the beak ; on the mesial lobe, in ad- 

 dition to this, there are other smaller ribs intercalated. 



Fig. 6 a, b. Two individuals of this species in green shale. 

 Fig. 6 c. The surface enlarged, showing the concentric strise. 

 Fig. 6 d. A cast of the beak of the same. 



Position and locality. In the upper green shale at Sodus, associated with Lepteena corrugata 

 and Spirifer radiafus ; and in the -same position at Rochester and intermediate localities. 



459. 30, ATRYPA ROBUSTA. 



PL. XXIII. Fig. 7 a, b, c, d. 



Subrhomboidal, trilobate ; surface marked by eleven or twelve strong angular plications on 

 each valve, four of which are raised on the mesial fold, and three in the depression of the op- 

 posite valve ; plications crossed by abruptly undulating striee ; cardinal line not extended, and 

 the sides sloping abruptly from the beak. 



The specimen figured is somewhat compressed, the perfect form being more rotund. The 

 species is readily distinguished by the small number of strong angular plications, which are 

 undivided from the beak. In the specimen figured, the beak of the dorsal valve is broken off 

 at the termination of the beak of the ventral valve ; beyond -vvhich it extended some distance, 

 as usual in species of this form. The concentric striae in the specimen figured are nearly ob- 

 literated by exfoliation of the shell. 



