Plate I. 



Goniatites crenistria Phillips, Lower Carboniferous, Arkansas. 



Fig. 1, a-j, development, shown in septa, from protoconeh to ma- 

 turity. 

 Fig. 2, early larval stage, diam. 0.47 mm. 

 Figs. 3 and 4, larval stage, diam, 0.92 mm. 

 Figs. 5 and 6, adolescent stage, diam. 1.29 mm. 

 Figs. 7-9, adult shell and septa. 



A highly specialized Goniatite, and a representative of the group 

 radicle of the Tropitidae and Arcestidae among the Mesozoic Ammonites. 



Gastrioceras Listeri Martin, Coal Measures, Arkansas. 



Figs. 10-11, adult shell. 



A still more highly specialized Goniatite, showing further progress 

 toward the Tropitidae. 



Gastrioceras Branneri Smith, Coal Measures, Arkansas. 

 Figs. 12-14, adult shell and septa. 



Aganides rotatorius de Koninck, Lower Carboniferous, Indiana. 



Figs. 15 and 16, adult shell. 



The genera illustrated on this plate show the stage of evolution of 

 the common Carboniferous groups, and the early ancestral types of the 

 Arcestidae and the Tropitidae. 



All figures on this plate are from J. P. Smith, Carboniferous Am- 

 monoids of America, Mon. XLII, U. S. Geological Survey, 1903. 



