RHYNCHONELLIDiE OF THE CHEMUNG GROUP. 349 



Rhynclionella (Stenocisiua) stephani. 



PLATE LV. 



Shell varying from subtriangular-ovate, with length, and width about 

 equal, to rotund or subglobose, with width greater than length ; more 

 or less deeply sinuate in front. 

 Ventral valve depressed-convex or but moderately convex above, curving 

 gradually to the lower lateral margins and more abruptly to the car- 

 dinal slope, where it is a little concave ; becoming depressed in a 

 broad undefined sinus, which begins at about one-third to one-half the 

 length from the apex ; curving gradually in young shells, and very 

 abruptly in older ones, to the elevated front of the dorsal valve : beak 

 slightly incurved, except in the older and more gibbous specimens. 

 Dorsal valve prominently convex and regularly arcuate from beak to 

 base in young and full-grown shells, but becomes extremely gibbous 

 in old shells, with a prominent fold on the anterior half of the valve. 

 Surface marked by about twenty-eight to thirty-two angular plications, 

 of which four, five or six are depressed in the mesial sinus and a cor- 

 responding number elevated on the mesial fold. The sinus and fold are 

 not distinctly limited, but a smaller and less elevated plication bounds 

 the fold and sinus on either side. 



The specimens described are casts or exfoliated shells which do not 

 preserve the finer surface markings. The casts show the impression of 

 short dental lamellae in the ventral valve and a strong mesial septum in 

 the dorsal valve, reaching somewhat more than one-third the length of 

 the shell. 



This species bears considerable resemblance in form and proportions to the 

 R. (8.) eximia, but is more angular in its outline and more coarsely plicated. 



Geoloffical formation and localities. This species occurs in the Chemung group 

 near Ithaca, associated with E. {S.) eximia ; at Philipsburgh in Allegany county, 

 and also in Cortland county, New- York. It is found near Bedford, Pennsylvania, 

 whence I have received some specimens from Dr. R. P. Stkvens. 



