ORTHIDES OP THE UPPER HELDERBERG ROCKS. 



Genus Ortiiis ( Dalman). 

 orthides of the upper helderberg group. 



There are three, or perhaps more, species of Ortiiis 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 Helderberg 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 diSerent 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 

 [ Paleontology IV.] 5 



