LAMELLIB RANCHI A fA. 
3 
individuals have a height of 80 mm. with nearly equal length, and a hinge¬ 
line of 50 mm. The gradation 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. pedeniformis 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. scabridus, 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 probable 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. pedeniformis , 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. at.: 
