688 



NORTH AMERICAN INDEX FOSSILS. 



262. 0. cyrtolites McChesney. (Fig. 975.) Mississippic. 



Slender, smooth, with apex curved over strongly ; dorsally sub- 

 angular. 



Burlington of Illinois, Iowa, and Missouri. 



263. 0. chesterensis M. and W. 



**QT 



% 



Mississippic. 



Fig. 974. Orthonychia formosutn, at- Fig. 975. Orthonychia cyrtolites. 



tached to crinoid. (After Keyes. ) (After Keyes.) 



Small, strongly curved, with about five grooves and very sin- 

 uous aperture. 



Chester of Illinois, Missouri, and Kentucky. 

 264. 0. acutirostris Hall. (Fig. 976, 977.) 



Mississippic-Carbonic. 



Apex making about a single volution, not in contact ; shell 



FlG. 976. Orthonychia acutirostris on Pterotocrinus acutus. ( After Keyes. ) 



widening abruptly below apex, often with shallow plications ; 

 aperture subcircular with sinuate margin. 



Keokuk and Warsaw of Illinois, Indiana, and Alabama ; St. 



Fig. 977. Orthonychia acutirostris. (After Whitfield, Mus. Comp. Zool. Bull. 



