Plate VI. 



Metasihirites Frechi Hyatt and Smith, Upper Triassic, California. 



Figs. 1-10, adult stage, showing arrest of development, reversion to 

 the Goniatite stage, and many characters of genera that came 

 between the primitive Goniatites and the more specialized Am- 

 monites in the history of this stock. 



Leconteia calif ornica Hyatt and Smith, Upper Triassic, California. 



Figs. 11-13, adult stage, showing arrest of development, and rever- 

 sion to the primitive ancestral type. 

 Figs. 14-15, larval stage, diam, 2.5 mm. 



Homerites semiglohosiis Hauer, Upper Triassic, California. 



Figs. 16-21, adult stage, showing arrest of development, and rever- 

 sion toward the ancestral stock. Both Homerites and Metasihi- 

 rites show a tendency to develop a rudimentary keel, probably 

 as a convergence phenomenon. 



Paraganides calif ornicus Hyatt and Smith, Upper Triassic, California. 



Figs. 22-26, adult stage, showing arrest of development, and rever- 

 sion to the Goniatite characters. 



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

 pod Genera of America. 



