124 



NORTH AMERICAN INDEX FOSSILS. 



Fig. 1367. Oncocerns pandion, ventral and lateral views, and section. 

 (After Clarke, Pal. Minn.) 



Stones River of Wisconsin and Minnesota. 



169. 0. exiguum (Billings). (Fig. 1368.) Ordovicic. 

 Small, short, slender; aperture somewhat abruptly constricted. 



Living chamber long, camerse high. 

 Trenton of Minnesota and Canada. 



170. 0. orcas (Hall). (Fig. 1369.) Siluric. 



Fig. 1368. Oncoceras exiguum. (After 

 Clarke, Pal. Minn.) 



Fig. 1369. Oncoceras orcas ^ with 

 shell partly removed, X %- (After 

 Hall, 20th Mus. Rep.) 



With very long, gradually contracting living chamber. 

 Niagaran of Wisconsin. 



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