343 
PALEONTOLOGY OF NEW-YORK. 
cavity, and margining the upper part of the muscular area, which is 
o£ten distinctly limited by the rostral cavity. In the dorsal valve there 
is a distinct septum reaching half the length of the valve, its inner 
upper edge partially divided, and leaving an angular prominence which 
has been the filling of the depression in the septum. 
The surface plications appear to be more angular in specimens from 
the arenaceous beds; and the casts of the interior of some specimens, 
which I have referred to this species, present distinctly angular 
plications. 
This species is readily distinguished from R. (S.) sapjoho by its more rotund 
or ovoid form and more rounded plications. The ventral valve is more convex, 
and never so flat at the sides ; while the ends of the plications, curving upward 
to meet the margin of the dorsal valve, form a distinctive feature, apparent in the 
profile views. In this respect, it bears more resemblance to some of the rotund 
forms of the Lower Helderberg group, as for example the R. jpy rami data. 
In some of its conditions as casts in a ferruginous argillaceous sandstone, this 
species bears much resemblance to the R. (/S'.) contracta of the Chemung group ; 
but that species is never so rotund, the sinus is deeper, and the plications are 
angular. 
Mr. Conrad’s description of Atrypa congregata is as follows : “ Suborbicular, 
“ with about fifteen rounded costas crossed by wrinkled lines ; lesser valve with 
“ the central part flat, slightly elevated, except towards the base, where it is more 
“ prominent, wide at base and rapidly narrowed above, with four flattened ribs ; 
“ inferior valve with a regularly concave depression in the middle. Locality. Conk- 
“ lin’s Falls, near Apulia, Onondaga county.” 
This species is spoken of as “the most abundant fossil in a formation of shale ” 
(Table of formations, No. 22) ; and since the above described species of Rhyn- 
chonella is the only one which is at all common in the neighborhood of the 
locality named, I have referred it to Mr. Conrad’s species. 
Geological formations and localities. In the condition of casts of the interior, 
this species occurs abundantly in some calcareo-arenaceous or partially shaly bands 
of the Hamilton group, near Fultonham and Summit in Schoharie county, at the 
Unadilla forks in Otsego county and near South Onondaga. It is found in the 
Tully limestone near Tinker’s falls. A single specimen, of small size, is from a 
calcareous band in the Hamilton group at Alden in Erie county. 
I have received several very good specimens from Rev. E. J. Bush of Hamilton 
in Madison county. 
