^6 NORTH AMERICAN INDEX FOSSILS. 



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'a ^' 



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Fig. 1290. Mitroceras gebhardi, side and umbilical view of a nearly perfect internal 

 mold, not showing the septa however, X %■- (After Grabau.) 



Ryticeratida. 

 XXVII. ZiTTELOCERAS Hyatt. 

 Cyrtoceracones with depressed, elliptical or ovate section venter 

 narrower than dorsum; lines of growth frilled, alternately fine 

 and coarse. Ordovicic-Devonic. 



81. Z. hallianum (D'Orbigny). (Fig. 1291.) Ordovicic. 

 Arcuate, with somewhat distant, strongly frilled, growth lines, 



and short camerse. 



Black River of Minnesota and Wisconsin, and Trenton lime- 

 stone of New York and Canada. 



82. Z. billingsi (Salter). Ordovicic. 

 More strongly arcuate than preceding, shell more rapidly ex- 

 panding, growth lines more crowded and less strongly frilled. 



Stones River of Minnesota, Canada, and the Cincinnati region. 



83. Z. nereus Hall. Devonic. 

 Large, strongly curved, adult less so; strong concentric frills 



with finer ones between; section subcircular. 

 Onondaga of New York, etc. 



XXVIII. Halloceras Hyatt. 

 Gyroceracones of subtrigonal section, the young like the adult 

 of Zitteloceras; venter broadest, dorsum subangular; typically 



