Plate X. 



Neolobites Choffati Plyatt. 



Fig. 1, showing arrest of development and reversion of a Cretaceous 

 genus to the Paleozoic Goniatite stage. 



Heterotissotia neoceratites Peron, Upper Cretaceous, Peru. 



Figs. 2-4, convergence with the Triassic Ceratites, by reversion to 

 some ceratitic ancestor, though probably not Ceratites. 



Ceratites semipartitus v. Buch, Middle Triassic, Germany. 



Fig. 5, septa for comparison with the "Pseudoceratites" of the 

 Cretaceous. 



Otoceras Woodiuardi Diener, Lower Triassic, India. 



Fig. 6 and 7, a transitional Permian and Lower Triassic genus, to 

 show heterochronous convergence with some of the "Pseudo- 

 ceratites" of the Cretaceous. 



Paratissotia serrata Hyatt, Upper Cretaceous, Peru. 



Figs. 8-10, a Cretaceous genus, arrested in development, and show- 

 ing atavistic reversion to characters very like those of Otoceras 

 of the Permian and Lower Triassic, 



Sphenodiscus Hilli Hyatt, Upper Cretaceous, Texas. 



Fig. 11, septa, showing resemblance to Arcestidae of the Triassic, 

 though probably not indicating relationship. 



Waagenoceras Hilli Smith, Permian, Texas. 



Fig. 12, septa, showing resemblance to those of Spenodiscus of the 



Cretaceous — a case of heterochronous convergence. 

 Figs. 1, 8, 9, 10, 11, from Hyatt, Pseudoceratites of the Cretaceous. 

 Figs. 6 and 7, from Diener, Cephalopoda of the Lower Trias. Mem. 



Geol. Survey, India, 1897. 

 Figs. 2-5, from Steinmann, Probleme der Ammoniten-Phylogenie. 



Sitz. Niederrhein. Gesell. Bonn, 1909, 

 Fig. 12, from J. P. Smith, Carboniferous Ammouoids of America. 



