RHYNCHONELLID^ OF THE CHEMUNG GROUP. 3S3 



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 Cortlaudville, 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. 



Rliynchonella (Stenocisma) orbicularis. 



PLATE LV. 

 RkynchotuUa orbicularu : 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 

 [ Pal.£ontologt IV.] 45 



