No. 109.] 7! 



This species belongs to the type of R. icihoni, aud may be said to be 

 one step farther removed from that species than R. nucleolata; or, in other 

 words, bearing about the same relation to the latter species, which that 

 one does to R. wilsoni. 



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

 derberg group, Albany county. 



Rhynchonella VELLICATA. 

 Pal. N.y. Vol. iii, pi. 33, f. 1 a to A. 



Shell varying from transversely oval to subtriangular : ventral 

 valve depressed-convex; beak somewhat prominent, depressed, 

 closely incurved over the opposite : dorsal valve more gibbous ; 

 beak incurved, not prominent. Surface marked by twenty-four 

 to thirty-six plications, six to eight of which are elevated in front 

 of the dorsal valve so as to form a rather.distinct mesial promi- 

 nence, rarely extending beyond the middle of the valve. On the 

 ventral valve, five to seven of the plications are depressed, to- 

 wards the front, into a more or less distinctly defined sinus, and 

 prolonged, forming a mesial projection, which is more or less 

 elevated in the front of the other valve. Near the junction of 

 the valves in front, very fine closely arranged lines of growth 

 are visible. 



This shell approaches very nearly, in some of its characters, the Rhyn- 

 chondla abrwpta : there arc, indeed, some forms which it is difficult to 

 distinguish. In the well-characterized specimens of this species, it differs 

 from R. ahrupta in its smaller and more numerous plications, and in being 

 proportionally less ventricose, as well as in the narrower and deeper sinus 

 of the ventral valve. The general aspect of the shells is usually quite dis- 

 tinctive. 



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

 derberg group, Albany aod Schoharie counties. 



