CEPHALOPODA— AMMONOIDEA. 



207 



adult and old age; the final portion generally straight or with a 



final crook ; ornamentation by costse and often by nodes. Cretacic. 



360. H. (Didymoceras) newtoni Whitfield. (Fig. 1476.) Cretacic. 



Helicoceran portion low, with simple and bifurcating costae, and 



^' ^\i 



Fig. 1476. Heteroceras {^Didymoceras) newtoni^ helicoceran fragment, X !> ^i^^ 

 septum, X f • 



two rows of tubercles, one just above the line of contact of the 

 whorls. 



Pierre of the Black Hills. 



361. H. (Didymoceras) tortum (M. and H.). (Figs. 1477-1479.) 



Cretacic. 

 Large, volutions nearly doubling in diameter with each turn, 



Fig. 1477. Heteroceras {^Didymoceras) tortum, peripheral view of a fragment (type), 

 (After Meek.) 



disconnected, with umbilicus less than diameter of largest volu- 

 tion; surface with two rows of rather depressed nodes, passing 



