76' [Senate 



elevated bifurcating plications on each valve, about six of which 

 are very slightly elevated near the front of the dorsal valve, so 

 as to form a broad flat indistinct mesial prominence, correspon- 

 ding to a faint shallow sinus in the opposite valve. 



A single imperfect specimen only of this species has come under my 

 observation. The regularly arched dorsal valve. ( very slightly elevated ), 

 flattened mesial fold, depressed ventral valve, and bifurcating strias will 

 probably serve to distinguish it from all the allied forms found in our rocks. 



Geological position and locality. Shaly limestone of the Lower Hel- 

 derberg group, Albany county. 



Rhtnchonella SULCOPLICATA. 

 Pal. N.Y. Vol.iii, pi. 35, f. 1. 



Shell subtriangular, wider than long, compressed : valves nearly 

 equal ; beak of the ventral valve prominent, attenuated, acutely 

 pointed and arched ; beak of the dorsal valve incurved. Surface 

 marked by about sixteen simple longitudinally grooved plica- 

 tions, four of which are' very faintly elevated towards the front 

 of the dorsal valve, forming an indistinct mesial. fold, corre- 

 sponding to a sinus in the opposite valve which is occupied by 

 three plications. 



A marked peculiarity of this species is the longitudinal groove along the 

 centre of each of the plications. Of the four plications elevated on the 

 dorsal valve, the two central ones are less prominent than the others, and 

 separated by a deeper and wider depression which continues quite to the 

 apex of the beak. 



Geological position and locality. Shaly limestone of the Lower Hel- 

 derberg group, Albany county. 



RhYNCHONELLA FORMOSA. 



Pal. N.Y. Vol. iii, pi. 35, f. 6. 



Shell subtriangular or transversely oval ; lateral margins forming 

 an angle at the beak of about 90° to 110« ; ventral valve some- 



