126 PALAEONTOLOGY OF NEW YORK. 



Left valve moderately convex, depressed holow. Riffht valve siiiallcr .iihI 

 less convex, in'arly flat hclow. 



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



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

 valve. T'^nibonal region moderately gil)l)()us in the Ii'l't valve, sulitending an 

 acute angle. 



Ear rounikMl, a little ol)Ui|ue, defined liy a nearly vertical depression. In 

 the right valve the ear is Hat or concave, liinitcil liy the abrupt elevation 

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

 length dl' the body, and in tlic right valve joining the body at the posterior 

 end, without distinct limitation in cither valve : margin moderately concave ; 

 extremity slightly |Mddnced, angular. 



Surface of left valve niarkeil ly sharp, elevated, Lunellose, concentric 

 stria^, with intermediate liner lines; and by slender, elevated rays, with wider 

 interspaces between the primar\' rays, wliich ai'e often oc('n]iied with one or 

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

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

 concentric lines, wliich are .slightl}' curved backw'ard. The surface of the 

 right valve is marked by strong, lamellose, concentric strije, which liecome 

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

 of this valve are marked by numerous fine rays. 



Interior uid<nown. 



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

 4S mm., height .'!() Tum.. and binge-line :]'] mm. 



This species resembles A. eta, but differs in the more extemU'(l wing, finer 

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

 more numerous concentric lamelhe. 



Formation and localitij. in tlie shales of tiie lowei- part of tlie ('lienumg 

 group at Ithaca, N. Y. 



