72 



[ Senate 



Rhynchonella altiplicata. 



'^ Pal. N.Y. Vol.iii, pi. 33, f.2. 



Shell snbtrigonal, more or less gibbous : ventral valve depressed- 

 convex J beak pointed, arched or nearly straight : dorsal valve 

 the larger, most elevated in the middle, declining with a curved 

 outline towards the beak and margins ; beak incurved ; foramen 

 triangular, extending to the apex of the beak. Surface marked 

 by from ten to about nineteen simple, strongly elevated, sharply 

 angular plications on each valve ; two to four of which are 

 elevated on the dorsal valve into a more or less distinct mesial 

 prominence extending nearly to the beak, and from one to three 

 depressed on the middle of the ventral valve into a distinct 

 sinus, which widens regularly and somewhat rapidly from near 



s the beak to the front, where it is prolonged into a short projec- 

 ; . tion, filling a corresponding sinus in the front of the opposite 

 valve : shell traversed by fine concentric lines of growth. 



' Along the lateral slopes of the cardinal margin, on each side of the beaks, 

 there is generally an oval space of greater or less extent, not plicated. This, 

 although sometimes slightly concave, is never so distinctly impressed as 

 often in species of the type of R. vjilsoni. 



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

 derberg group, Albany and Schoharie counties. 



1, 2, 3, 4 : R. ALTIPLICATA. 6, 6 : R. tbansvkrsa. 7 : R. acutiplicata» 



