RHYNCHONELLID^ OF THE HAMILTON GROUP. 345 



arcuate from beak to base along the gently depressed mesial sinus, 

 which bears four plications in the bottom, and one on each side par- 

 tially included, giving it an undefined outline. 



Dorsal valve gibbous, somewhat regularly convex in the middle and 

 upper part, and interrupted only on the lower third by the moderately 

 elevated mesial fold ; the plications on the lower lateral margins 

 strongly curving to the edge of the ventral valvel 



Surface marked by about thirty subangular plications on each valve ; 

 of which four and partially a fifth are depressed in the sinus, giving 

 five prominent and a sixth less elevated on the mesial fold. The finer 

 markings are not preserved in the specimen examined. 

 From a partial cast of the apex of the ventral valve, it appears to 



have had short dental lamellae. 



This species bears much resemblance in its general form to R. (S.) Carolina of 

 the Comiferous limestone ; but it is more rotund, the sinus less wide and not 

 reaching so far towards the beak, the plications less angular and more numerous. 

 I have a single specimen only, received from Rev. E. J, Bush, and this one has 

 the appearance of being an adult shell, measuring nearly three-fourths of an inch 

 in lenorth and having about the same width. 



Geological formation and locality. In the Hamilton gi'oup, near Hamilton, 

 Madison county, New- York. 



The following species from the Tully limestone is given in this connexion on 

 account of its geological position and relations, although it does not appear to me 

 to be quite congeneric with the preceding and following sijecies. Leaving it under 

 the Genus Rhtnchoneixa for the present, I nevertheless believe that when its 

 internal characters shall be fully known, this one and a few others will constitute 

 a distinct genus. In its young condition this species has much the aspect of Leio- 

 KHTNCHU8, and might readily be mistaken for a species of that genus. 



[ Pal^ontoloot IV.] 44 



