RHYNCHONELLIDiE 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 thalia, being 
preoccupied, must give way to another. 
Geological formation and localities. In the Corniferous limestone of Western 
New-York and Canada West. 
Rliyiiclionella (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 about 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 
[Paleontology IV.] 
43 
