3-l«  PAL.llONTOLOGY OF NEW-YORK. 



cavity, and margining the upper part of the muscular area, which is 

 often 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 distiiigiiisbed from R. {^8.) sappho by its more rotund 

 or ovoid form and more rounded pb'cations. The ventral valve is more convex, 

 and never so flat at the sides ; while the ends of the plications, ciUTing 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. pyramidata. 



In some of its conditions as casts in a ferruginous argillaceous sandstone, this 

 species bears much resemblance to the R. {8.) contractu of the Chemung group ; 

 but that species is never so rotund, the sinus is deeper, and the plications are 

 angular. 



Mr. Coxrad's description of ^^ry^a coM(7re^ato is as follows :. " Suborbicular, 

 " with about fifteen rounded costte 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 Avith 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 Ehyn- 

 CHONELLA is the only one which is at all common iu the neighborhood of the 

 locality named, I have referred it to Mr. Conrad's species. 



Geological fonnations and localities. In the condition of casts of the interior, 

 this species occurs abundantly in some calcareo-arcnaccous 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 

 TuUy limestone near Tinker's fails. 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. Bcsn of Hamilton 

 in Madison county. 



