988 PALiEONTOLOGY OF NEW-YORK. 



numerous casts in my collection. Mr. Davidson says: "No defined septum ia 

 " 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 of 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 difiercnces, 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. coucentn'ca is a characteristic fossil of the Upper Devonian strata of 

 Europe, but Mr. DAvrosoN 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 ; which 

 would leave us to infer that it is not common in that position. Our species begins 

 its existence at the epoch of the Coraiferous 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 Cornifcrous 

 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 AVidder station and Bosanquet in Canada West, in the same posi- 

 tion. It likewise occm-s in Pennsylvania, Maryland and Virginia; principally, how- 

 ever, in the condition of casts of the interior. From Cumberland, Md., I have 

 received ppecimcns from Mr. Andrews, as well as from the collections of Mr. E. 

 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. 



