LAMELLIBRANCHIATA. 119 



Left valve regularly convex, moderately gibbous at the umbo. Right 

 valve depressed-convex below, more convex above. 



Hinge-line straight, length equal to the length of the body. 



Beaks sub-anterior, in the left valve prominent and rising alcove the hinge ; 

 in the right valve the beak extends to the hinge-line. Umbonal region 

 moderately gibbous in the left valve, convex in the right, subtending nearly 

 a right angle. 



Ear short, separated from the body by a marked byssal depression ; extrem- 

 ity rounded. Wing narrow, triangular, joining the body midway between the 

 beak and the posterior extremity, not distinctly defined ; margin concave ; 

 extremity produced and acute. The right valve is less distinctly auriculate 

 in front ; the anterior extremity acute ; the wing is larger and the margin 

 less abruptly concave. 



Test thin, marked l)y fine concentric stritv which are more conspicuous and 

 equidistant on the right valve. The left valve is marked with fine, closely- 

 arranged radii, which are strongest on the upper margin of the wing. The 

 body of the right valve shows only sharp, distant, concentric stria? ; the wing- 

 is marked by a few rays in the lower part, and by stronger radii along the 

 hinge-line, which are cancellated by the concentric stria?. The auricle is 

 radiated, and cancellated in the same manner. 



Interior unknown. 



A left valve has a length of 13 mm., height 10 mm., and hinge-line 12 mm. 

 A large right valve has a length of 15 mm., height 1"! mm., and hinge-line 

 15 mm. 



Tliis species differs from A. tenuistriata in its more oblique l)ody, compara- 

 tively longer hinge-line, and more closely arranged and stronger radii. 



Formation and locality. In the lower part of the Chemung group, near 

 Ithaca, N. Y. 



