Brachiojioda of tJie Cincimiati Group. 17 



Genus Rhynchonella — (Fischer, 1809). 



Shell trigonal, acutely beaked, usually plaited ; dorsal valve elevated 

 in front, depressed at the sides ; ventral valve flattened or hollowed 

 along the center ; hinge plates suj)porting two slender, curved lamellse ; 

 dental plates diverging. 



Khynchonella capax — (Conrad, 1842). 



Shell medium size, varying with age from compressed subtrigonal 

 to large subglobose, old examples being often more convex than 

 their diameter in any other direction ; posterior lateral margins some- 

 what straightened and converging to the beak, at about a right angle 

 in young shells, but becoming more rounded in the adult ; lateral 

 margins rounding to the front, which is more or less distinctly sinuous, 

 or nearly straight in the middle. 



Dorsal valve generally a little more convex than the other, most 

 prominent in the middle, and rounding abruptly, or sloping more 

 gently from the central region in all directions ; the more elevated 

 part forming, anteriorly, a depressed mesial ridge, that is nearly flat, and 

 occupied by four plications on top, and rarely continues two thirds of 

 the way to the strongly incurved beak, while on young or compressed 

 individuals it is faintly marked even anteriorly ; lateral slopes each 

 occupied by four to seven or eight simple, angular plications. 



Ventral valve with its beak abruptly pointed, and very strongly in- 

 curved upon that of the other valve, in adult shells, but less distinctly 

 curved, and showing a small opening under its apex, in young ex- 

 amples ; mesial sinus deep and well defined in gibbous specimens, and 

 less so in the young or more compressed forms, never quite reaching 

 the point of the beak, and always having three simple, rather angular 

 plications in the bottom, that extend, like the others, to the apex of 

 the beak, in well preserved specimens; lateral slopes each occupied 

 by from five to seven simple i^lications. 



Entire surface of both valves marked by numerous, very regular, 

 strongly zigzag, prominent, sublaminar marks of gro'wth, that become 

 nearly or quite obsolete, sometimes, on old examples. 



Its range is confined to the rocks constituting the upper 250 feet of 

 the Cincinnati Group, but it is the most abundant and common fossil 

 within that range, and one of the best preserved. It can be picked up 

 almost anywhere north of the Ohio river, from 40 to 60 miles from the 

 city of Cincinnati. 



