LOWER HELDERBERG ROCKS. 
fi *$5 
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 different 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 between the Hamilton and Chemung 
groups, as also between these groups and the rocks of the Carboniferous 
system. 
IJngula 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. . 
Surface marked by concentric lines of growth and finer lamellose con¬ 
centric strise, 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 reaching from the beak quite to the base, and the very equidistant 
lamellose strife upon 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. 
