L26 PALEONTOLOGY OF NEW YORK. 



Left valve moderately convex, depressed below. Right valve smaller and 

 less convex, nearly Mat below. 



Hinge-line straight, length nearly equal to the height of the shell. 



Beaks Bub-anterior, prominent, arching over the hinge-line in the left 

 valve. CTmbonal region moderately gibbous in the left valve, subtending an 

 acute angle. 



Ear rounded, a little oblique, denned by a nearly vertical depression. In 

 the right valve the ear is Hat or concave, limited by the abrupt elevation 

 of the umbo. Wings large, in the left valve extending about three-fourths the 

 length of the body, and in the right valve joining the body at the posterior 

 end, without distinct limitation in either valve ; margin moderately concave; 

 extremity slightly produced, angular. 



Surface of left valve marked by sharp, elevated, lamellose, concentric 

 striae, with intermediate finer lines; and by slender, elevated rays, with wider 

 interspaces between the primary rays, which are often occupied with one or 

 two finer lines; the ear and wing are marked in a similar manner. The 

 rays are distinctly crenulated, and the interspaces are cancellated by the 

 concentric lines, which are slightly curved backward. The surface of the 

 right valve is marked by strong, lamellose, concentric stria?, which become 

 less conspicuous on the wing. The posterior slope of the body and the wing 

 of this valve are marked by numerous fine rays. 



Interior unknown. 



A specimen, with the two valves attached, has approximately a length of 

 IS mm., height 30 mm., and hinge-line 33 mm. 



This species resembles A. eta, but differs in the more extended wing, finer 

 radii of the left valve, with interstitial additions, narrower interspaces, and 

 more numerous concentric lamellae. 



Formation and locality. In the shales of the lower part of the Chemung 

 group at Ithaca, N. Y. 



