No. 109.] 77 



what more depressed than the opposite ; beak prominent, arched, 

 not strongly incurved : dorsal valve larger, declining with a 

 gentle curve towards the margins j beak incurved. Surface 

 marked by twenty to twenty-four simple angular plications on 

 each valve, from two to four of which in the middle are coarser 

 and depressed in the ventral, having a corresponding number 

 abruptly elevated upon the dorsal valve. 



This handsome species may be recognized by the neatly rounded outline 

 of the latero-basal margins, the abrupt sinus, and the stronger central 

 plications. 



A single specimen from the same position as the above, shows a les3 

 distinctly defined sinus and mesial elevation, with five plications on the 

 latter and four in the former, and only six on each side. I am disposed to 

 regard this as only a variety of the above, though future collections may 

 prove it to be distinct. Figure 2 a, h, c and d of the same plate are given 

 to illustrate this form. Figures 3 and 4 a, b, c, d, e, f, of the same plate, 

 illustrate specimens with from two to four plications on the mesial eleva- 

 tion, and from four to six on each side. 



Geological positioji and locality. Shaly limestone of the Lower Helder- 

 berg group, and the Upper Pentamerus limestone of Albany and Schoharie 

 counties. 



RhTNCHONELLA rOEMOSA. 



