126 
PALEONTOLOGY OF NEW YORK. 
Left valve moderately convex, depressed below. Right valve smaller and 
less convex, nearly flat below. 
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. Umbonal region moderately gibbous in the left valve, subtending an 
acute angle. 
Ear rounded, a little oblique,.defined by a nearly vertical depression. In 
the right valve the ear is flat 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 striae, 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 
48 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. 
