36 NORTH AMERICAN INDEX FOSSILS. 



' . Ribs coarse, alternating in size ; tubercles not 



strong in adult CLI. Douvilleiceras. 



' . Ribs broken by spirals into numerous uniform 



nodes CXII A, Trachysagenites. 



' . Ribs few, coarse, a double row of strong ventro- 

 lateral tubercles CLII. Metoicoceras. 



12/ . Ribs not extending across the venter k''. 



k''. Discoidal, with large umbilicus ; end of costse 



tubercled , CXLII. Aspidoceras. 



k^. Compressed, with small umbilicus 4^. 



4:|:. With sharp median geniculation of ribs 

 marked by angulation on groove. 



CXXXVII. CEcotraustes. 

 4j. Without pronounced geniculation. 



CXXXVI. Oppelia. 



23f. Ribs bifurcating or trifurcating above umbilicus r. 



r. Surface of shell without nodes 13^. 



13''. Umbilicus large 1'. 



Y. Section transverse 5j. 



5 J. Ribs crossing venter uninterruptedly. 



* CXL. Perisphindes. 

 5 J. Ribs bending forward on venter and becom- 

 ing obsolete in center a' . 



a^ . Young coronate, of trapezoidal section, 

 CXLV. Aulacostephanus. 



a\ Young not coronate CXLI. Idoceras. 



V . Section laterally compressed, whorls high...6j. 



6J. Ribs bifurcating. ..CXLIII. Olcostephanus. 



6J. Ribs dividing into bundles of three or four. 



CXLIV. Virgatites. 



13^. Umbilicus small CXXXVIII. Macrocephalites, 



r. Surface of shells with nodes or tubercles at division or 



at end of branches or both 14-'. 



14^. Ribs interrupted ventrally m''. 



m^. With median ventral channel. 



CXLVI. Hoplites. 

 m^. With flattening, but no channel, nodes weak. 

 CXLIX. Lyticoceras, 



1 4''. Ribs continuous across venter n''. 



n^. Ribs enlarging ventrally, with intercalated 



smaller ribs CXLVII. Stoliczkaia. 



n^. Ribs continuous and strong from umbilical 



tubercle CXLVIII. Sonneratia. 



uu. Surface with coarse folds, tubercles and rather definite ribs ; 

 young with compressed, narrowly-channeled venter ; suture 



highly complex CLX VIII. Stantonoceras. 



3333. Close coiled throughout, with venter acute, angulated, flat, 

 channeled or modified by keel which is often bordered by 



grooves vv. 



vv. Surface strongly ribbed 24f. 



