LAMELLIBRANCHIA TA. 
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 above 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 by fine concentric strise 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 striae ; 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 striae. The auricle is 
radiated, and cancellated in the same manner. 
Interior unknown. 
A left valve has a length of 13 mm., height 10 min., and hinge-line 12 mm. 
A large right valve has a length of 15 mm., height 12 mm., and hinge-line 
15 mm. 
This species differs from A. tenuistriata in its more oblique body, 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. 
