106 PALAEONTOLOGY OF ILLINOIS. 



Position and locality: Coal measures, Rolls Ford, Sangamon 

 county. 



Collector, A. H. Worthen. 

 No. 2597 of the Illinois State Museum. 



GENUS ORTHIS. Dalman. 



; 



ORTHIS RESUPINOIDES. Cox. 



PI. XI, Figs. 4-4a,4b. 



Shell transversely ovate to subquadrate in outline, hinge line 

 straight, and scarcely more than half as long as the greatest 

 breadth of the shell; cardinal angles rounded, cardinal area 

 narrow, and widest on the dorsal valve. 



Dorsal valve depressed, convex towards the beak, and strongly 

 depressed at the front, giving a sinuous outline to the frontal 

 margin. 



Ventral valve convex, the greatest convexity rather nearer 

 the beak than the front. Beaks depressed, incurved, and pro- 

 jecting slightly beyond the hinge line. 



Surface of the valves marked by numerous fine thread-like 

 stride, and several transverse lines of groAvth. 



This shell is so closely allied to Martin's species, O. rexupinata,, 

 that it can hardly be regarded as specifically distinct. The only 

 external differences observable, on a comparison of the American 

 with authentic examples of the European shell are, the compara- 

 tively shorter hinge line and somewhat finer striae of the Ameri- 

 can shell. 



Position and locality: Lower Coal Measures, Mercer county, 

 Illinois. 



Collector, Tyler McWhorter, Esq. 



No. 2600 of the Illinois State Museum. 



