Plate XII. 



Cordillerites atigtdatus Hyatt and Smith, Lower Triassic, Idaho. 



Figs. 1-8, development from larval stage to maturity. A primitive 

 Ammonite, showing simple recapitulation; a very perfect repe- 

 tition of phylogeny in ontogeny. 



Pronorites cyclolobus Phillips, Lower Carboniferous, England. 



Fig. 9, showing development of the septa. The three species of Pro- 

 norites illustrated are examples of the ancestral stock of Cor- 

 dillerites and Medlicottia. 



Pronorites mixolobus, Carboniferous, England. 



Fig. 10, septa, for comparison with P. cyclolohus. 



Pronorites cyclolobus, var. arkansasensis Smith, Lower Carboniferous, 

 Arkansas. 



Figs. 11 and 12, shell and septa. 



Figs. 1-8, from Hyatt and Smith, Triassic Cephalopod Genera of 

 America. 



Figs. 9-12, from J. P. Smith, Carboniferous Ammonoids of America. 



