LOWER HELDERBERG ROCKS. 837 



d of the same plate are given to illustrate this form. Fig. 3 and 4 «, b, c, d, e,f,of 



the same plate, illustrate specimens with from two to four plications on the mesial 



elevation, and from four to six on each side. One specimen from the shaly limestone 



of Becraft's mountain shows a single sharp plication in the mesial sinus, and two 



elevated upon the opposite valve. 



Fig. 6 a - e. Ventral, dorsal, profile, and front views of several individuals of the typical 

 form, with strong plications. 



Fig. 6f- I. Specimens with finer and more numerous plications. 



Fig. 6 w - y. Illustrations of specimens which present the ordinary variations of full-grown 

 individuals, from the Upper Pentamerus limestone. 



Geological position and locality. In the shaly limestone of the Lower Helderberg 

 group, and in the Upper Pentamerus limestone : Helderberg mountains, Schoharie, 

 Hudson, Catskill, Carlisle, Cherryvalley, and other places. 



Rliynclionella eniinens (n. s.). 



Plate XXXVII. Fig. 3 & 4. 



Shell abruptly ovoid or depressed subglobose. Dorsal valve the larger, 

 elevated near the front into a flattened mesial prominence, from which 

 it declines gently towards the beak and more abruptly towards the 

 lateral margins, which are deflected towards the opposite valve : beak 

 incurved. Ventral valve flattened, very abruptly deflected at the mar- 

 gins towards the opposite valve, having a broad well-defined sinus 

 reaching from near the middle to the front, and prolonged into a lingui- 

 form extension. 



Surface marked by about twenty-six rounded or scarcely subangular 



plications, about six of which are on the mesial fold and five in the 



opposite sinus; the whole crossed by fine undulating or zigzag lines of 



growth near the front of the valves. 



Sometimes the plications bounding each side of the mesial fold in this species 



bifiu-cate; and one of them becomes obsolete before reaching the front, as in R. 



abrupta and R. vellicata. From both of these species, it may be distinguished by its 



more rounded plications and more prominent mesial fold, as well as in being a more 



robust shell. 



Fig. 3 a, b. Dorsal, front, and profile views of an individual of medium size. 



Fig. 4 a. Dorsal valve of a larger individual. 



The species sometimes attains larger size than fig. 4. 



Geological position and locality. In the shaly limestone of the Lower Helderberg 

 group : Helderberg mountains, Albany county. 



