LAMELLIBRANCHIATA. 3 



individuals have a height of 80 inin. with nearly equal length, and a hinge- 

 line of 50 mm. The gi'adation from this form is very gradual to those in 

 which the height is equal to, or greater than, the length, and where the 

 length of the hinge-line is nearly equal that of the shell. 



This species differs from A. pecteniformis by the larger anterior ears, with 

 more numerous and less prominent rays on the body of the shell. In specimens 

 identified as A. Sanduskyensis the umbo is more inflated and obtuse, while the 

 length is uniformly greater than the height. A. scahridus, which in some 

 respects resembles this species, is clearly distinguished by its characteristic 

 surface-ornamentation and the proportionally greater height of the valves. 



This is one of the largest and the most abundant species in the lower and 

 middle portions of the Hamilton group. It is very easily distinguished by the 

 large ears with the furrowed ligamental area, full, obliquely ovate outlines, and 

 characters of ornamentation. It first appears in the Upper Helderberg 

 limestone in western localities. It is abundant in the lower beds of the 

 Hamilton group, especially in the coarser shales of the eastern central part of 

 the State ; while it is rare in the upper soft shales of the group, and scarcely 

 ever seen in the western counties. 



Mr. Conrad suggests the prol)able identity of this species with the subse- 

 quently described form, A. parilis. A careful comparison of a large number 

 of specimens establishes this synonymy, and indicates the propriety of uniting 

 this form with the A. pecteniformis, Conrad. Aviculopeden Sanduskyensis, Meek, 

 which was afterwards referred by that author to A. parilis, Conrad ? belongs to 

 this species. 



The apparent specific differences noted in the description of these three 

 forms are neither constant nor well defined; nor even to be regarded as 

 marking varieties. They are separately considered for the purpose of referring 

 to the original descriptions, and to correspond with the original specimens, 

 which are figured for two of the species. Farther investigation will probably 

 show more satisfactory proof of the validity of the present conclusions. The 

 original description of Mr. Conrad is as follows — loc. cit. : 



