LOWER HELDERBERG ROCKS. 835 



Rliynclionella rudis ( n. s.)- 



Plate XXXIV. Fig. 20 & 21. 

 Shell triangular, wider than long ; lateral margins abruptly tapering to 



the beak. Ventral valve much depressed or flattened : beak scarcely 



incurved. Dorsal valve the larger, much 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 mesiai fold, and three depressed in the front of- the other 



valve. 



The surface of this shell was doubtless marked by concentric strise ; but the only 

 specimens I have seen are not sufficiently well preserved to retain them. 

 Fig. 20 a, b, c. Ventral, front and profile views. 

 Fig. 21 a, b. Profile and front view of a similar form, which varies in some respects. 



Geological position and locality. Upper calcareous part of the shaly limestone of 

 the Lower Helderberg group, Hudson. 



Rhynclionella planoconyexa ( n. s.). 



Plate XXXIV. Fio. 22. 



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 elevated plications on each 

 valve, several of which bifurcate towards the margin, and 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, corresponding 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 (which is moderately elevated ), the flattened 



mesial fold, depressed ventral valve, and bifurcating striae will probably serve to 



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



Fig. 22. A cast of the dorsal valve, preserving the shell upon the margins. 

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



group : Albany county. 



