LAMELLIBRANCHIATA. 5 



Hinge-line straight, having a length of more than two-thirds the length 

 of the shell. Umbo prominent, sloping rapidly to the cars; the Bides 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-halt' 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 striffi 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 mm. 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. 



