86 



NORTH AMERICAN INDEX FOSSILS. 



Fig. 1304. Nephriticeras viagister, X Yz- (After Hall, Pal. N. Y., V.) 



large, subcentral, expanding slightly between the septa; surface 

 with rounded or sharp undulating spirals, 5 or 6 in space of 5 mm. 

 Hamilton of New York; Falls of Ohio. 



XXXI. Stroboceras Hyatt. 



Gyroceracones or nautilicones of slight involution, with gibbous, 

 umbilical shoulders making the dorsum wider than the venter, and 

 with two pair of lateral, revolving ridges. Sutures with broad, ab- 

 dominal saddles, small, acute saddles at the lateral ridges, and nar- 

 row lobes on either side ; broad lateral saddles on swollen or gib- 

 bous part of whorl, and small, subacute, dorsal lobes ; aperture con- 

 tracted and dumb-bell shaped; siphuncle centroventran ; young 

 like Trigonoceras. Mississippic-Carbonic. 



100. S. trisulcatum (Meek and Worthen). (Fig. 1305.) 



Mississippic. 



With three concave sulci, or channels separated by sharp, ventral 

 ridges ; impressed zone slight. 



Rockford Goniatite bed of Indiana; Waverly of Ohio. 



