LOWER HELDERBERG ROCKS. ISS 



bed without the intervention of these sediments. These faunas, as they 

 occur in the southwest in the same group of beds, may be as distinct in 

 the order of time as those which are separated from each other by a 

 thousand feet in thickness of other deposits. 



This example of the apparent mingling in the same group of the faunas 

 of two distinct periods, is only one among numerous others that have been 

 observed among the dififerent groups constituting the Silurian, Devonian 

 and Carboniferous systems. We shall have occasion to notice similar 

 features between the Lower and Upper Helderberg groups in the absence 

 of the Oriskany sandstone, and betAveen the Hamilton and Chemung 

 groups, as also between these groups and the rocks of the Carboniferous 

 system. 



Lin^la centrilineata ( n. s.). 



Plate IX. Fig. 1, 2. 



Shell oval-ovate, about once and a half as long as wide : beak obtuse : 

 base rounded, greatest width central or a little below the centre, very 

 little convex. 



SuBFACE marked by concentric lines of growth and finer lamellose con- 

 centric striae, which are nearly obliterated by the exfoliation of the 

 outer shell, when the surface presents fine parallel longitudinal striae 

 and a central impressed line from beak to base. 



The characterizing features in this species, as far as can be observed, are the 

 central line reachiiig from the beak quite to the base, and the very equidistant 

 lamellose stride ujwn the exterior shell, which make a shorter curve than the con- 

 centric striae of growth ( these are not well shown in the figure). On the exfoliated 

 surface, the concentric striae are very faint or altogether obsolete. 



Fig. 1. A small individual of this species. 

 Fig. 2. A larger specimen of the same. 



Geological position and locality. In the shaly limestone of the Lower Helderberg 

 group : Helderberg mountains, Albany county. 



