FOSSILS OF THE NIAGAEA GROUP. 135 



Rhyxchoxella PISA (n. sp.). 



Plate 7, figs. 18-22. 



Shell small, globular in full-grown specimens, but varying from de- 

 pressed lenticular to highly gibbous at different stages of growth. Gen- 

 eral outline of the shell broadly ovate, widest near the front or below 

 the middle of the length. Ventral valve less gibbous than the dorsal ; 

 beak small, pointed, and slightly curved, usually projecting but mod- 

 erately beyond that of the opposite valve, though sometimes rather ex- 

 tended ; body and sides of the valve regularly rounded, becoming broadly, 

 but not deeply, sinuate in front, where it is prolonged and bent upward 

 in the middle. Dorsal valve regularly convex, the center of the valve 

 the most prominent, becoming slightly elevated in front, forming a not 

 very prominent mesial elevation. 



Surface marked by from twelve to sixteen rounded plications, which 

 are distinctly marked on the sides and in front, but become obsolete soon 

 after reaching the middle of the valve on the dorsal side, but are con- 

 tinued somewhat further on the ventral, and in some specimens those 

 bordering the sinus extend to near the beak. On the dorsal valve there 

 are usually four plications elevated to form the mesial fold, and three de- 

 pressed in the ventral sinus, though in some cases there are only three 

 elevated on ihe dorsal valve. 



The species resembles R. negleda, with which it is associated, but 

 differs in the general convexity of the valves and want of angularity in 

 the plications, which do not extend to the beak, as in that species. The 

 more elongate forms resemble R. acinus, Hall, but have not the flattening 

 of the sides and surfaces of the valves, as in that species, and have, 

 moreover, a greater number of plications. The more ventricose forms 

 resemble very closely small specimens of R. nucleolala, Hall, but, besides 

 being more elongate, with a more projecting beak, the plications are 

 never flattened on their surfaces, and are destitute of the groove along 

 the middle, as in that one. 



Formation and local ity : In the limestones of the Niagara group, near Danville and 

 Sinking Springs, Highland county, Ohio. State Collection. Collected by Prof. Ed- 

 ward Orton. 



