38 NORTH AMERICAN INDEX FOSSILS. 



WW. Form a loose spiral in young, the adult straight with a final 



recurving portion CLI V. Ancyloceras. 



WW. Form a succession of straight limbs connected by curved 



portions 26t. 



26f. Limbs distant, with continuous ribs, no tubercles. 



CLV. Hamites. 

 26t. Limbs in contact or even impressed ; ribs smooth or 



tuberculated CL VL Ptychoceras. 



m. Whorls coiled in a spire, which is more or less regular 4444. 



4444. Spire low, nearly in single plane ; surface ribbed and noded. 



XX. 



XX. Early whorls in contact ; ribs with three rows of tubercles. 



CLVII. Helicancyclus. 

 XX. Whorls more or less free, early ribs with two rows of costae. 



CLIX, Exiteloceras. 

 4444. Spire of medium height, with broad base, costse tubercled, 



young portion straight CLX A, Didymoceras. 



4444. Spire high, with ribs and nodes yy. 



yy. Whorls round, ribs extend from suture to suture, nodes few. 



CLVII L Helicoceras. 

 yy. Whorls subangular, with regular spiral rows of nodes on 



angulations CLXII. Turrilites. 



m. Whorls coiled irregularly or straight 5555' 



5555. Early whorls in low spiral followed by long, straight portion 

 with costse, often bifurcating and sometimes tuberculated. 



CLXI. Anisoceras. 

 5555. Early whorls high-spired, growth in later stages very irregular, 



generally represented only by fragments CLX. Heteroceras. 



5555. Coiled only in the youngest stages, all parts generally seen 



straight and smooth CXXXII. Baculites. 



B. DiBRANCHiATA : Mostly naked cephalopods, well represented in modern faunas 

 (squid, cuttlefish, octopus, etc.) ; shell when present internal, often enclosed in 



a calcareous " guard." Animal with two gills II. 



II. Belemnoidea : Camerated shell (phragmocone ) enclosed in a calcareous 



guard 3. 



3. Phragmocone long, guard short CLXXIV. Atractites. 



3. Phragmocone much shorter than the guard 7*. 



7*. Guard with short, deep ventral furrow below alveolar margin, shell end- 

 ing in mucronate point CLXXVI. Belemnitella. 



7*. Guard with long furrow, or smooth, without mucronate terminal. 



CLXXV. Belemnites. 

 II. Sepioidea : Shell (internal) ending posteriorly in a mucro, the anterior por- 

 tion expanded into a broad proostracum 4. 



4. Proostracum long, thickened by calcareous lamellae ; mucro minute Sepia. 



4. Proostracum not known, mucro large, expanded CLXXVII. Belosepia. 



