LOWER HELDERBERG ROCKS. 
S3 S 
Rhynclionella radls (n. s.). 
Plate XXXIY. Fig. 20 & 21. 
Shell triangular, wider than long; lateral margins abruptly tapering to 
the beak. Yentral 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 mesial fold, and three depressed in the front of the other 
valve. 
The surface of this shell was doubtless marked by concentric striee; but the only 
specimens I have seen are not sufficiently well preserved to retain them. 
Fig. 20 a, b, c. Yentral, front and profile views. 
Fig. 21 n, 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 Llelderberg group, Hudson. 
Rliynchonella planoconvexa (n. s.). 
Plate XXXIY. Fig. 22. 
Shell subcircular or transversely oval. Yentral 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 strirn 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 I-Ielderberg 
grpup : Albany county. 
