64 NORTH AMERICAN INDEX FOSSILS. 



Black River of Minnesota and the Cincinnati region; Black 

 River and Trenton of Canada; Trenton of New York. 



55. S. thoas (Hall). (Fig. 127 1.) Devonic. 



Annulations straight, round, and distant, interspaces concave; 

 resembles Dawsonoceras annulatum but is without the frills and 

 has continuous fine striae. 



Schoharie and Onondaga of New York and Ohio. 



{ 



Fig. 1 27 1. Spyroceras thoas, y^%. (After Hall.) 



56. S. crotalum (Hall). Devonic. 

 Annulations sharper than in preceding, sometimes oblique or 



undulating; camerse irregular, deeply concave; longitudinal strias 

 fine. 



Hamilton of New York; Columbus of Ohio. 



57. S. nuntium (Hall). (Fig. 1272.) Devonic. 

 Differs from the preceding in the closer set, horizontal annula- 

 tions, and more pronounced, longitudinal striae. 



Hamilton of New York. 



