Plate IV. 

 Columbites parisianiis Hyatt and Smith, Lower Triassic, Idaho. 

 Figs. 1-3, adult stage. 

 Figs. 4 and 5, adolescent stage, diam. 10 mm., corresponding to 



Gastrioceras. 

 Figs. 6 and 7, easily adolescent stage, diam. 1.75 mm. 

 Figs. 8-10, embryonic and early larval stages. 



Columbites shows a transition from the group of Gastrioceras towards the 

 Tropitidae. 



Tropites subbullatus Hauer, Upper Triassic, California. 



Figs. 11-13, adult stage. 



Figs. 14-15, adolescent stage. 



Figs. 16-21, larval stages, showing development from the Goniatite 

 to the Ammonite stage, with very unequal acceleration, "tele- 

 scoping" of characters and stages. 



Columbites is the Lower Triassic ancestor of the Tropitidae, and connects 

 that family with the Paleozoic ancestors, Glyphioceratidae. 

 All figures on this plate are from Hyatt and Smith, Triassic Cephalo- 



pod Genera of America, Prof. Paper No. XL, U. S. Geological Survey, 



1905. 



