FOSSILS OF THE NIAGARA GROUP. 417 



cave interspaces, which, on the larger part of the shell, are distant 

 fully three-sixteenths of an inch and gradually converge towards 

 the apex. The shell is silicified so that the finer markings are 

 obscured, but there are indistinct transverse striae crossing the 

 spaces between the ridges. No fine longitudinal striae have been 

 observed, though they may have existed on the shell in its original 

 condition. 



The similarity of this species to 0. columnar e is obvious, but it tapers 

 more rapidly, and the septa are less distant. The length of the specimen 

 figured is about eight and a half inches. 



The species has been illustrated in this connexion from its general 

 external similarity to figs. 4, 6, 8 and 9, and from a belief, originally, 

 that it was from the same horizon, or not far removed therefrom. The 

 specimen has been a long time in my possession, and was obtained in the 

 northwestern lake region many years since, but I have no means of 

 ascertaining the particular locality. The weathered surface of the lime- 

 stone has the aspect of the Niagara limestone ; but the fresh fracture 

 more resembles some beds of the Clinton group, in the vicinity of Green- 

 bay. A critical examination of the rock, the nature of the crystalline 

 filling of some of the cavities, together with the silicification of the 

 exterior shell and the principal part of the interior, sejDta, &c., induces 

 me to refer the species, with some hesitation, to the Lower Silurian age. 



Cab. Nat. 53 



