214 



besides the central one, which is divided into two or 

 three smaller ones (a generic feature). Sometimes there 

 is a broad, undefined depression, and frequently only a 

 narrow depression caused by the subdivided central pli- 

 cation. The ventral valve has uniformly a longitudinal 

 sinus, which includes two or three small plications arising 

 from the subdivision of the central one, and sometimes 

 including one or two on each side. 



The shell is marked by radiating rounded or sub-angular 

 costse or plications, from eight to twelve on each side of the 

 central one ; those on the cardinal slopes sometimes bifurcat- 

 ing, or with interstitial additions; while in a few individuals 

 bifurcating costas occur on other parts of the valve ; the 

 interspaces are rounded grooves of about the same size as 

 the plications. The surface is marked by fine concentric 

 strise, and stronger, imbricating, lamellose lines of growth. 



This species differs from R. (Atrypa) aprinis of the Niagara 

 group of New York, in being more ovate in form and less 

 gibbous towards the front, while the ventral beak is more 

 elevated. It approaches very nearly in character to the R. 

 formosa of the lower Helderberg group, and the shell is sub- 

 ject to similar variations of form, proportions and incurv- 

 ation of the beak, which in some specimens of both species 

 is closed upon the opposite beak. The central plication in 

 both species is divided into two or three, and the dorsal 

 valve likewise presents sometimes a mesial elevation and 

 sometimes a depression. 



The largest individual measured has a length of more 

 than seven eighths of an inch, with a width nearly as great. 

 The specimens vary from an eighth of inch to nearly one 

 inch in length. This species is comparatively numerous. 



