RHYNCHONELLIDiE OF THE CHEMUNG GROUP. 
353 
Geological formations and localities. This species occurs in the Chemung group, 
both in the shales and shaly sandstones of the central part of the group, and also 
in ferruginous sandstones and conglomerates of the upper portions, and in nume¬ 
rous localities from Tioga county to the western limits of the State. It is found 
near Owego, near Cortlandville, at Ithaca, on Cayuta creek, at Chemung narrows, 
Elmira, Painted-post, Bath, Angelica, Rockville, Philipsburgh ; near Olean, near 
New-Albion ; near Westfield, French creek, Howard’s on Chautauqua creek, 
Twenty-mile creek and elsewhere ; also at Meadville, Pennsylvania, from all of 
which places I have specimens before me. I have likewise received specimens 
from Dr. R. P. Stevens, collected at Bradford, Pennsylvania. Specimens from 
Licking county, Ohio, have the same aspect, and present all the modifications of 
form, varying number of plications on fold and sinus, and other characters marking 
the New-York specimens. 
Rliynclionella (Stenocisma) orbicularis. 
PLATE LY. 
Rhynchonella orbicularis : Hall, Thirteenth Report on the State Cabinet, p. 88. 1860. 
Shell suborbicular, ventricose on the dorsal valve; width greater than 
the length. 
Ventral valve moderately convex, scarcely gibbous on the umbo ; beak 
incurved; outline gently curving on the lateral and baso-lateral mar¬ 
gins, and the centre gently depressed in a broad and strongly defined 
median sinus. 
Dorsal valve gibbous in the middle, arcuate from.beak to base; sides 
abruptly curving to the ventral valve; median fold beginning above 
the middle of the length, and becoming more prominent towards the 
front. 
Surface marked by about twenty-four or more subangular or rounded 
ribs, those of the central portion being strong and elevated, and those 
towards the cardinal slope gradually finer and less distinct. There are 
four or five plications in the mesial sinus, and from four to six on 
the mesial fold, which are moderately elevated and gently curving 
[ Paleontology IV.] 45 
