78 [ Senate 



Rhynchonella eminens. 



Pal. N.Y. Vol. iii, pi. 37, f. 2. 



Shell abruptly ovoid or depressed subglobose : dorsal valve the 

 larger, elevated near the front into a flattened mesial prominence, 

 from which it declines gently towards the beak and more ab- 

 ruptly towards the lateral margins, which are deflected towards 

 the opposite valve ; beak incurved : ventral valve flattened, 

 very abruptly deflected at the margins towards the opposite 

 valve, having a broad well defined sinus reaching from near the 

 middle to the front, which is prolonged into a linguiform ex- 

 tension. Surface marked by about twenty-six rounded or scarce- 

 ly subangular plications, about six of which are on the mesial 

 fold and five in the opposite sinus ; the whole crossed by fine 

 undulating or zigzag lines of growth near the front of the valves. 



Sometimes the plications bounding each side of the mesial fold in this 

 species bifurcate, and one of them becomes obsolete before reaching the 

 front, as in R. abrupta and R. vellicata. From both of these species, it 

 may be distinguished by its more rounded plications and more prominent 

 mesial fold. 



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

 derberg group, Albany county. 



Rhynchonella ventricosa. 



Pal. N.Y. Vol. iii, pL43, f. 1. ■ 



Shell varying from spheroidal to vertically ovoid, extremely 

 ventricose ; depth of the valves often nearly double that of the 

 breadth ; valves nearly equal : dorsal valve sometimes a little 

 depressed in the umbonial region ; beak incurved ; cardinal 

 region on each side of the beak elevated : ventral valve having 

 a long linguiform extension ; beak rather small and obtuse, 

 closely incurved upon the opposite, subangular along its lateral 

 slopes. Surface marked by fourteen to twenty rounded plica- 

 tions, three or four of which are sometimes very sligbtly elevated 



