80 [ Senate 



undefined mesial fold, declining near the beak and curving down 

 abruptly at tlie sides ; beak incurved : ventral valve compressed, 

 abruptly deflected towards the opposite valve at the lateral 

 margins, depressed into a broad rounded sinus which occupies 

 almost the entire breadth of the narrow front ; front margin 

 curving upward, and extended into a triangular prolongation. 

 Surface marked by twenty-two or twenty-four simple rounded 

 subangular plications, five 6r six of which are elevated on the 

 mesial fold, and four or five occupy the sinus of the ventral 

 valve. Fine zigzag lines of growth are seen on the front of the 

 shell, near the junction of the valves. 



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

 derberg group, and in the succeeding ^^Scutella limestone^' of the same 

 group, Albany county. 



Rhynchonella NOBILIS. 



Pal. N.Y. Vol.iii, pi. 43, f. 3. 



Shell varying from compressed-ovate to subrhomboidal, becoming 

 in adult specimens broad-ovate and much more gibbous : dorsal 

 valve the larger, elevated in front into a somewhat rounded 

 mesial prominence which rarely extends beyond the middle of 

 the shell, declining laterally with an abrupt curve to meet the 

 inflected edges of the opposite valve ; beak incurved : ventral 

 valve depressed, (in old specimens) abruptly deflected at the 

 margins towards the opposite valve, depressed towards the front 

 into a shallow rounded mesial sinus, sometimes prolonged into 

 a vertical extension with nearly parallel sides ; beak small, not 

 prominent, incurved. Surface marked by twenty six to thirty- 

 two elevated angular plications, six to eight of which are ele- 

 vated on the mesial fold of the dorsal valve, and five to seven 

 depressed in the sinus of the other valve. Fine closely arranged 

 zigzag lines of growth may be seen near the margins of the 

 valves in front. 

 This species holds an intermediate position between JR. abrupta and R. 



suhcontracta : it is, however, always more elongated than the first, and 



