ORTHIDES OF THE UPPER HELDERBERG ROCKS. 
33 
Genus Obtiiis ( Dalman). 
ORTHIDES OF THE UPPER HELDERBERG GROUP. 
There are three, or perhaps more,' species of Orthis occurring in the 
Schoharie grit, which are the lowest beds of the group ; but since the 
specimens are almost invariably in the condition of casts of the interior, 
a satisfactory determination of their specific characters is attended with 
many difficulties. It has been only by preserving the casts of the interior, 
with such impressions of the exterior surface of the shell as could be 
obtained, that the characteristic features of two species have been pretty 
well ascertained. One of these occurs in considerable numbers, while the 
other is extremely rare : the first resembles a species found in the lime¬ 
stone above the grit, but known only in few examples; while the other 
is quite distinct from any known above the horizon of the grit. A third 
species, of peculiar characters, has been seen only in a single specimen of 
the cast of the ventral valve ; while some impressions of the exterior of 
a shell have not been identified with either of them, 
In the limestones above the Schoharie grit we have at least five well 
marked species, in addition to those just mentioned. 
The Orthides of this group of strata do not present any important 
differences, in general features, from those of the group below : they are, 
however, far less numerous, both as species and individuals. Two of the 
species are of the type of Orthis oblata ; and two others are of the type of 
O. subcarinata of the Lower Helderber’g group, while Orthis propinqua of 
the Upper Helderberg limestone resembles Orthis multistriata of the Upper 
Pentamerus limestone of the same group. 
In the details of form, surface markings, area, and muscular impressions 
of these different species, there are sufficient means of identification, and 
of distinction from those before described. 
Some of the species in the limestones of this group have a wide geo¬ 
graphical distribution, but no one of them has proved to be of common 
[ Palaeontology IV.] 5 
