12 PALEONTOLOGY OF NEW YORK. 



Valves convex ; the right valve less convex and smaller than the left valve. 



Hinge-line straight, less than the length of the valve. 



Beak aeiite, prominent, inclined forward, close to the anterior end of the 

 shell, rmbonal region prominent, and in the left valve gibbous; subtending 

 an acute angle. 



Ear small, separated from the valve by a broad sulcus, beyond which it is 

 a iHcrc liild ill the shell. Wing large, triangular, flat, extending nearly to 

 the margin of the valve; limited, in the left \;ilve, liy a inore or less dis- 

 tinct sulcus, and the abrupt bending of the concentric strias ; margin con- 

 cave; extremity acute. This character of the wing is somewhat less marked 

 in the right valve. 



Test thick ; the left valve marked with strong, prominent, rounded radii, 

 regularly alternating with finer ones on the posterior half of the valve; 

 crossed at regular intervals by strong concentric lamellae ; the inter.spaces 

 marked l)y fine lines of growth. From maceration or exfoliation these 

 surface characters are usually only partially preserved, or nearly obsolete. 

 In certain conditions the rays become nodose where crossed by the lamellae, 

 and in many examples the concentric striae interrupt the rays, leaving them 

 alternating above and below the lamella}. (See pi. xviii, figs. 13, 14.) 

 The surface characters of the body are continued on the wing, while on 

 the ear the concentric striae are ci'owded and conspicuous, and the rays 

 obsolete. In the right valve the markings are much subdued, the rays often 

 obsolete, especially on the lower part of the valve, and the concentric Innielhv 

 are simple undulations of the surface. 



Ligamental area narrow, linear ; marked, apparently, by a single groove. 

 The cast shows a fold corresponding to the sulcus limiting the wing. Pallial 

 line extending from just anterior ami below tlie uinlio to the posterior portion 

 of the body and thence recurving, terminating in a small oval nuiscular impres- 

 sion. No evidences of teeth are seen in the casts or the interior of valves. 



A large spt'cimen lias a length of 7ti mm., height 56 mm., hinge-line 62 

 mm. A medium sized example has a length of -52 mm., height 43 mm., 

 hinge-line 42 mm. 



