No. 109.] 75 



which from three to four are elevated near the front into a rather 

 faint mesial fold, and from two to three depressed on the ventral 

 valve so as to form a faint sinus in the front. Somewhat strong 

 zigzag lines of growth mark the surface of the valves near the 

 margin in front. 



There is another shell associated with the above, which agrees so very 

 nearly with it, that I am unwilling, without a better series of specimens 

 for comparison, to regard as distinct, though it differs somewhat in the 

 number of plications ( See f. 17 - 19, same plate). 



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

 derberg group, Albany county. 



Rhynchonella RUDIS, 



Pal. N.Y. Vol. iii, pi. 34, f. 21. 



Shell triangular, wider than long ; lateral margins abruptly ta- 

 pering to the beak : ventral valve much depressed or flattened ; 

 beak unknown : dorsal valve the larger, most elevated in front, 

 declining towards the beak, which is slightly incurved. Surface 

 marked by about ten rather distant plications on each valve, 

 three of which are elevated near the front of the dorsal valve 

 into a prominent mesial fold, and three depressed in the front 

 of the other valve. 



The surface of this shell was doubtless marked by concentric striae ; but 

 the only specimen I have seen is not sufficiently well preserved to retain 

 them. 



Geological position and locality. Upper calcareous part of the Shaly 

 limestone of the Lower Helderberg group, Hudson. 



Rhtnchonella PLANOCONVEXA. 



Pal. N.Y. Vol. iii, pi. 34, f. 2. 



Shell subcircular or transversely oval : ventral valve depressed- 

 convex, most prominent near the beak : dorsal valve larger, 

 regularly convex. Surface marked by about twenty-four sharply 



