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PALAEONTOLOGY OF NEW-YORK. 
numerous casts in my collection. Mr. Davidson says': “No defined septum is 
“ observable in this [the dorsal] valve, but a rudimentary mesial ridge divides 
“ the quadruple impressions of the adductor.” In the “ ventral valve the dental 
“ plates are tolerably developed ; while the adductor leaves a small oval scar 
“ towards the middle of the valves, and which scar is separated into two parts by 
“ a minute mesial elevation, under and outside of which are seen the large impres- 
“ sions of the divaricator .muscle.” 
In our specimens there is a thin defined septum in the dorsal valve, reaching 
from the apex for more than half the length o£the shell; and in the ventral valve 
the dental plates are rather strongly developed, while the lower margin of the 
adductor impression is considerably above the middle. These differences, there¬ 
fore, do not serve to change the opinion already expressed, and I shall retain for 
the present the name originally proposed by Eaton ; more especially since it has 
priority in point of time. 
The A. concentrica is a characteristic fossil of the Upper Devonian strata of 
Europe, but Mr. Davidson remarks that it is not very common in Great Britain ; 
and he says he has not seen it from the Middle Devonian of Newton and Ogwell 
in Devonshire, whence it has been cited by Mr. Morris in his Catalogue ; w'hich 
•would leave us to infer that it is not common in that position. Our species begins 
its existence at the epoch of the Corniferous limestone, and is abundant in the 
Hamilton group, but I do not know of its occurrence in the Chemung group. It 
must rank, therefore, rather as a Lower than an Upper Devonian species in 
America. 
Geological formations and localities. This species occurs in the Corniferous 
limestone in Albany and Schoharie counties, and rarely in the same rocks in the 
western part of the State. It is common in the Hamilton group, along Cayuga 
lake, and at localities east of that line, though more often occurring as partial 
casts. It is likewise common on the shores of Seneca and Canandaigua lakes, in 
the Genesee valley at several localities, at Darien, Hamburgh and Eighteen-mile 
creek, being more abundant in the latter place than any other locality known to 
me. It occurs at Widder station and Bosanquet in Canada West, in the same posi¬ 
tion. It likewise occurs in Pennsylvania, Maryland and Virginia; principally, how¬ 
ever, in the condition of casts of the interior. From Cumberland, Md., I have 
received specimens from Mr. Andrews, as well as from the collections of Mr. R. 
P. Whitfield ; and I am indebted to Dr. G. A. Williams for many fine casts and 
a few specimens retaining the shell, from Hardy county, Va. 
