L12 PALEONTOLOGY OF NEW YORK. 



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

 Binge-line straight, less than the length of* the valve. 

 I leak acute, prominent, inclined forward, close to the anterior end of the 

 shell. Umbonal 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 mere fold in the shell. Wing large, triangular, flat, extending nearly to 

 the margin of the valve; limited, in the left valve, by a more or less dis- 

 tinct sulcus, and the abrupt bending of the concentric stria?; 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 liner ones on the posterior half of the valve: 

 crossed at regular intervals by strong concentric lamellae; the interspaces 

 marked by 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 stria? interrupt the rays, leaving them 

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

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

 the ear the concentric stria? are crowded 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 lamellae 

 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. Pallia! 

 line extending from just anterior and below the umbo to the posterior portion. 

 of t he bod} and thence recurving, terminating in a small oval muscular impres- 

 sion. Xo evidences of teeth are seen in the casts or the interior of valves. 



A large specimen has a length of 7G mm., height 56 mm., hinge-line 62 

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

 hinge-line VI nun. 



