FOSSILS OF THE SILURIAN AND DEVONIAN ROCKS. 75 



Rhynchonella Carolina. HALL. 



Plate XIII., figures 1, 2 and 3,"34 and 35, 



Rhyn. Carolina, Hall. Pal. N. Y., Vol. IV., p. 337, pi. 34, figs. 14-191867. 

 Rhyn. Carolina, Meek. Pal. of Ohio, Vol. I., p. 196, pi. 18, figs. 8a-e 1873. 



Shell of medium size, trigonoid-subovate in general outline, moderately 

 gibbous, a little produced or widened in front. In an old shell of unusual size 

 belonging to my cabinet, the widening of the front portion is very consider- 

 able, so that the lateral margins, from beak to a little below the middle of the 

 shell, become strongly concave, while in younger specimens, those as figures 34 

 and 35, said margins are almost straight. 



Ventral valve a little less convex than the other ; greatest convexity at the 

 umbo, from where it curves gently towards the front, and more abruptly to 

 the cardinal margins; beak prominent, little curved or nearly straight, and 

 elevated a little above umbo of other valve. 



Mesial sinus starts a little above the middle of the valve, is broad, shallow 

 and flat at the bottom ; its margins are not well defined. Dorsal valve more 

 convex, moderately gibbous and regularly arcuate from summit to base, the 

 sides more abruptly curved ; mesial fold, beginning a little above middle of 

 valve, is broad, slightly convex, mostly flat, without well defined margins ; 

 beak strongly incurved into the other valve. 



Surface is marked by from twenty-four to thirty simple, sub-angular, radi- 

 ating costae, of which from six to eight occupy the mesial sinus, and about 

 the same number the mesial elevation ; the ribs in the sinus and on the fold 

 are stronger than the lateral ones. The plications increase from the beak 

 towards the front in size and distance, and the lateral ribs are curving more or 

 less outwards. No other surface-markings are visible, not even concentric lines 

 of growth, on th most perfectly preserved shells in my cabinet. This species 

 differs from all other rhynchonelloid forms of the Corniferous limestone, by 

 its broad, shallow and poorly defined sinus, as well as by its broad, flat and 

 also poorly defined mesial elevation, and by its more extended beak. Prof. 

 Hall is of the opinion that a larger material may hereafter prove its generic 

 distinction from Rhynchonella. 



Formation and Locality. Occurs in the Corniferous limestone at and near the Falls of the Ohio, 

 in Kentucky and Indiana. It is rather rare, though I possess in my cabinet a few exceedingly well pre- 

 served specimens retaining the shell entirely. 



