208 



NORTH AMERICAN INDEX FOSSILS. 



i 



around below middle of outer side, and smaller annular costse 

 sometimes bifurcating at nodes. 



Pierre group, near Fort Pierre, Dakota. 



Fig. 1478. Heteroceras {Didymoceras) tortum, opposite view ot specimen, Fig. 1477. 

 (After Meek.) 



362. H. conradi Morton. (Fig. 1480.) Cretacic. 



Mostly U-shaped fragments ; more or less subcircular in section, 



or compressed in one direction; ribs sharply angular; a double 



row of more or less strongly developed nodes, either lateral in 



Fig. 1479. Heteroceras {Didymoceras) tortum, suture enlarged. (After Meek, 



position or upon outer side of U-shaped tube ; ribs often obsoles- 

 cent between nodes. 



Ripleyan (Navesink) of New Jersey. 



CLXI. Anisoceras Pictet. 

 Young shells helicoceran followed by long, straight (toxoceran) 

 portion; costse often bifurcating, simple or with two rows of 

 tubercles; gerontic stage with retroversal crook. Cretacic. 



