136 Carl 0. Dunbar, 



in the upper layers of the shale on the lower course of Big Sandy 

 River. 



Eatonia fissicosta, n. sp. 

 Plate III, figs. 7, 8 



Description: Shell planoconvex, transversely ovate in outline. 

 Ventral valve varying from slightly concave to gently convex 

 as a whole, with a sharper convexity for a short distance below 

 the beak. A narrow sinus begins near the middle of the valve, 

 widening gradually and becoming deep at the front, where it 

 forms a linguiform extension. The sinus is bounded by sharp 

 ridges on either side, which correspond to depressions on each 

 side of the fold of the opposite valve. Hinge-line slightly 

 declining from the beak, which is small and neatly incurved, hid- 

 ing the deltidium. The beak is perforate as in E. tennesseensis. 

 A well developed false cardinal area is present on each side of the 

 beak. Dorsal valve gibbous. The high and narrow fold does 

 not reach to the beak. Ventral muscular impression essentially 

 similar to that of E. tennesseensis. Surface with many rather 

 fine angular plications which rapidly increase by bifurcation. A 

 stronger one usually occupies the center of the sinus. The rate 

 of bifurcation is irregular, and the number of plications increases 

 with size, but an average specimen has eight on the fold and 

 twelve on each lateral slope. This character of bifurcating pli- 

 cations, to which the name alludes, is sufficient to distinguish the 

 species from any other known Eatonia. 



Dimensions of a medium specimen: Length, 16 mm.; width, 

 18 mm. ; depth, 12 mm. One very large specimen almost doubles 

 these measurements. 



Occurrence: Common in the Rockhouse shale at the Rockhouse 

 locality on Horse Creek. 



FAMILY CENTRONELLID^ 



Oriskania condoni (McChesney) 

 Plate HI, figs. 16, 17 

 Rcnssekcria condoni McChesney, New Pal. Fossils, 1861, p. 85; 

 Trans. Chicago Acad. Sci., 1868, p. 36, pi. 7, fig. 2. — Meek 

 and Worthen, Geol. Surv. Illinois, vol. Ill, 1868, p. 401, pi. 

 8, fig. 4. 



