LAMELLIBRANCHIATA. 251 



muscular impression on the post-umbonal slope. The interpallial area is 

 finely pustulose. 



This variety was originally separated from L. macroptera by reason of its 

 proportionally broader, shorter, and more gibbous form ; the arcuation of the 

 body of the left valve, and its expansion on the <antero-basal margin. In a 

 single example of the left valve, the radii are much stronger than usual in 

 typical specimens of the other form, and the interstitial radii more strongly 

 developed. These characters, and others upon which the species was originally 

 founded, when compared with a large number of individuals of the genus, are 

 represented with many modifications, and the specific distinction is here 

 continued with much reservation. 



On comparing the typical specimen of Mr. Conrad's L. macroptera, we find 

 it proportionally shorter and wider than many specimens since referred to the 

 same species ; measuring 63 mm. in length and about 78 mm. in height, the 

 left valve very gibbous and the body sub-arcuate ; the proportions being very 

 nearly the same as in L. curvata, represented in fig. 2, of plate xxviii. 



Formation and localities. This species occurs in the Hamilton group at 

 Cazenovia and other places in Madison county, N. Y. 



