34 NORTH AMERICAN INDEX FOSSILS. 



oooo. Shell faintly ribbed qq. 



qq. Whorls rounded, scarcely embracing I5-)-. 



I5|. Young whorls trapezoidal, ventrolateral angles noded. 



CXI, Columbites. 

 I5t. Young whorls depressed, but not trapezoidal. 



CV. Celtites. 

 qq. Whorls subquadrate, moderately embracing. 



CVI. Metaiirolites, 



oooo. Shell strongly ribbed rr. 



rr. Venter with central furrow i6f. 



i6"f. Ribs coarse, straight, strongly noded or spinous, whorls 



slightly embracing CIX. Clionites [Traskites). 



l6\. Ribs sharp, curving, nodes fine, in spiral row, whorls 



moderately embracing CX. Trachyceras {Anolciies). 



rr. Venter without furrow CVIII. Acrochordiceras. 



2. Suture in adult complexly lobed, the lobes and saddles notched or with 



secondary lobation (ammonitic) 6*. 



6*. Ventral saddle strongly notched, the others simple or with a single 



notch on each side XCV. Medlicottia. 



6*. Lobes and saddles simple in form, both slightly notched 1. 



1. Whorls compressed or flat nil. 



nil. Venter rounded ss] 



ss. Surface faintly ribbed lyf. 



lyf. Venter flat, but not furrowed. 



CII. Meekoceras {sens, strict^. 



lyf. Venter furrowed CX. Trachyceras {Protrachyceras^. 



ss. Surface strongly ribbed iSf. 



I Sf. Venter keeled n. 



n. Keel not flanked by depressed grooves. 



* CVIIA. Gymnotoceras. 



n. Keel flanked by depressed grooves. 



CXV. Paratropites. 

 i8f. Venter not keeled, ribs strongly cancellated. 



CXII, Sagenites. 

 ss. Surface smooth except for fine spirals. Umbilicus small or 



closed; venter narrowly rounded CXVIII. Ussuria. 



nil. Venter acute tt. 



tt. With high keel igf. 



i.9f . Surface with dichotomous sickle-shaped ribs. 



CXIV. Discotropites. 



I9f. Surface smooth CXXVII. Hauericeras. 



tt. Without keel ; surface smooth or costate 2of. 



2of. Young with venter grooved or channeled. 



CLXV. Metengonoceras. 

 2of . Young with venter not grooved or channeled. 



CLXVI. Sphenodiscus. 



tt. With ventral groove instead of keel 2if. 



2l|. Groove margined by smooth keels, surface smooth. 



CLXIII. Protengonoceras. 



2 if. Groove margined by rows of nodes, surface generally 



noded or ribbed CLXIV. Engonoceras. 



