LAMELLIBRANCHIA TA. 
5 
Hinge-line straight, having a length of more than two-thirds the length 
of the shell. Umbo prominent, sloping rapidly to the ears; the sides sub¬ 
tending a little more than a right angle. Beak prominent, situated a little 
anterior to the middle of the hinge. 
Ears triangular, unequal, the anterior about one-half the size of the 
posterior one, lateral margins concave. 
Right valve not observed. 
Test ornamented by about 35 sharp, strong rays, with intermediate finer 
ones, which are crossed by fine imbricating striae of growth. The radii are 
not present on the ears. Internal characters not shown in the specimen 
described. 
The original of this species has a length of 47 mm. with an equal height. 
The hinge-line measures 27 mim The specimen is somewhat imperfect at 
the extremities of the ears, and the posterior ear is not sufficiently extended 
in the figure. 
This form may be distinguished from A. princeps by its smaller anterior 
ear, and less expansion at the base of the ears. The umbo appears more 
contracted, and in the single known specimen is more prominent. The 
principal radii are sharper and stronger, and the alternation of smaller ones 
more regular. 
In A. exacutus, which resembles this form, the beaks are nearer the middle 
of the hinge-line, and the shell has a distinctly reticulate surface ornamenta¬ 
tion, with the radii continued over the ears. 
The specimen figured is the left valve, and is the original of Mr. Conrad’s 
description, no other specimen having been observed. 
Formation and locality. From the corniferous limestone of the Upper Helder- 
berg group, Schoharie, N. Y. 
