Plate XV. 



Clionites (Traskites) rohustus Hyatt and Smith, Upper Triassic, Cali- 

 fornia. 

 Figs. 1-8. A form arrested in development, and partly reversionary 

 to Trachyceras. 



Clionites (Calif ornites) Merriami Hyatt and Smith, Upper Triassic, Cali- 

 fornia. 

 Figs. 9-12. A form more retarded than C. rohustus, and showing 

 more of the ancestral characters. Reversionary, by arrest of de- 

 velopment, to Tirolites, in everything but the retention of the 

 trachyceran furrow. 



Trachyceras duplex Mojsisovics, Upper Triassic, Alps. 



Figs. 13-16. A progressive form, but showing the beginning of ar- 

 rest of development in the prolongation of the ontogeny, and 

 persistence of the Tirolites stage in adolescence. 



Clionites (Neanites) calif ornicus Hyatt and Smith, Upper Triassic, Cali- 

 fornia. 



Figs. 17-20. Eeversionary by arrest of development to the ancestral 

 type, Tyrolites, but still showing the trachyceran furrow in- 

 herited from its intermediate ancestor Trachyceras. 



Figs. 1-12, and 17-20, from Hyatt and Smith, Triassic Cephalopod 

 Genera of America. 



Figs. 13-16, from E. von Mojsisovics, Das Gebirge um Hallstatt, II, 

 1893. 



