66 [ Senate 



RhYNCHONELLA iEQUIVALVIS ( n. s.). 



Pal.N.Y. Vol.iii, pi. 29, f. 2. 



Shell ovate, somewhat compressed ; sides sloping from the beats 



at a little less than a right angle ; front semicircular ; valves 



nearly equally convex : ventral valve having sometimes towards 



the front a broad very faint depression or sinus ; beak pointed 



and incurved. Surface ornamented by twenty-eight to thirty - 



two simple rounded plications, broader than the depressions 



between. On the dorsal valve the central depression is a little 



deeper than the others, extending quite up to the beak : fine 



concentric lines, which arch a little upwards, cross the plications. 



This species may be compared with Terelratida haidiiigeri of Bar- 



EANDE, to some varieties of which it bears considerable resemblance : it 



is, however, generally less gibbous, and the beak of the ventral valve is 



much less prominent. The concentric undulations, marking the stages of 



growth, are likewise more faint than in Barhande's species. 



Geological position and locality/. Pentamerus limestone of the Lower 

 Helderberg group, Albany county. 



Rhynchonella mutabilis (n. s.). 



Pal. N.Y. Vol. iii, pi. 29, f. 4; and pi. 30, f. 1 & 2. 



Shell varying from ovate to spherical : ventral valve sometimes 

 depressed, generally most convex in the umbonial region; beak 

 small, pointed, closely incurved over that of the opposite valve : 

 dorsal valve gibbous ; beak incurved beyond the hinge-line ; 

 cardinal border on each side of the beak, concave. Surface 

 marked by twenty to twenty-six depressed, rounded, simple 

 plications, of Avhich about six or eight are slightly raised to- 

 wards the front of the dorsal valve into an indistinct mesial 

 elevation ; and five or six depressed near the front of the ven- 

 tral valve, and extended into a short linguiform prolongation, 

 fitting into a corresponding depression in the front of the oppo- 

 site valve. 



The plications on this shell are generally simple, though in a few spe- 

 cimens one or two of them are seen to bifurcate. The surface of perfect 



