Raymond: Pelecypoda of Chazy Formation. 335 



and lower slopes are gentle, but the front slope is abrupt, making an 

 angle of almost 90° with the plane of union of the valves. The surface 

 is marked by very fine concentric lines of growth. 



One specimen is 20 mm. long and 26.5 mm. high. 



Locality. — This species is found in the Lower Chazy at Chazy and 

 \'alcour Island, New York. 



Genus Ambonychia Hall. 



II. Ambonychia curvata Raymond. (Plate XXIX, figures 27, 28; 

 Plate XXX, figures 1-3.) 



Ambonychia? curvata Raymond, American Journal of Science (Ser. 4), Vol. XX, 

 1905. P- 373- 

 One of the most common pelecypods in the Chazy is a large form 

 which appears to belong to the genus Ambonychia, but as all the 

 specimens so far found are internal casts, there is no evidence as to 

 whether or not it had the characteristic radial striae of that genus. 



Description. 



Shell large, both valves very strongly convex, especially along the 

 region at the front and middle of the valves. Beaks small, incurved, 

 directed a little forward. Anterior slope abrupt and overhanging. 

 Posterior and basal slopes rather steep. Posterior wing short. The 

 posterior margin is slightly convex, almost straight, the anterior 

 margin rounded. 



The length and height are nearly equal. A large specimen is 27 

 mm. long and 26 mm. high. Another is 43 mm. long and 39 mm. 

 high. A small one is 10 mm. long and has the same height. The 

 species is easily recognized by the curved anterior margin and great 

 convexity. It is possible to confuse Clionychia montrealcnsis, C. 

 marginalis, and Ambonychia curvata, but C. montrealensis has the 

 greatest convexity along the middle of the valve; in C. marginalis 

 it is along the anterior margin, and in Ambonychia curvata the line of 

 greatest convexity is along a curve the convex side of which is forward. 

 Clionychia montrealensis also has much more narrow and depressed 

 beaks than the other two species. 



Locality. — All through the Chazy, especially in the upper part, 

 at Chazy, Valcour Island, and Sloop Island, New York. 



