KHYNCHONELLID^ OF THE CORNIFEROUS LIMESTONE. 337 



Dorsal valve convex ; mesial fold beginning above the middle of the 

 valve, and becoming prominent towards the front. 



Surface marked by about seventeen or eighteen sharply angular plica- 

 tions on each valve, of which three are in the sinus and four on the 

 mesial fold ; concentrically marked by fine abruptly undulating striae. 



The specimens which I have examined and referred to this species are somewhat 

 smaller than those figured by Mr. Billings. I am not quite satisfied that the spe- 

 cies is distinct from R. horsfordi of the Hamilton group ; but specimens of the 

 latter of the same size have a greater number of plications on the fold and sinus, 

 and the shell is larger. Should it prove a distinct species, the name thali'a, being 

 preoccupied, must give way to another. 



Geological fommition and localities. In the Corniferous limestone of "VYestern 

 New-York and Canada West. 



Rhynclionella (Stenocisma) Carolina (n.s.). 



PLATE LIV. 



Shell ovate, moderately gibbous, a little produced in front and broadly 

 sinuate ; length and breadth about equal. 



Ventral valve convex in the upper part, curving gently to the margins 

 and a little concave along the cardinal slope, sometimes nearly flat 

 below ; beak little incurved or nearly straight ; sinus beginning at 

 about one-third the length of the shell from the apex, very gradually 

 depressed and not abruptly incurved in front, making a broad shallow 

 sinus with curving sides, the limits of which are not strongly defined. 



Dorsal valve moderately gibbous and regularly arcuate from summit to 

 base, the sides more abruptly curved ; mesial fold becoming defined 

 above the middle of the shell, its summit convex and the sides not 

 abruptly limited. 



Surface marked by ^bout twenty to twenty-five obtusely angular plica- 

 tions ; those of the margins becoming obsolete and about four or five 

 depressed in the sinus, with a corresponding number on the dorsal fold, 

 which are stronger than the rest ; a single one on each side of the sinus 



[ Pal^ontoloot IV.] 43 



