1899.] 



PLEXODOXT MOL.\.US OF ifAMM-VLS. 



503 



The hypselodout molars with open cavity at the base, of several 

 Ungulates, e.g. the Toxodontia and Typotheria, show the greatest 

 departure from the primitive type by the complete fusion of their 

 elements ; however, by means of their oldest predecessors, they 

 can be traced to the same origin. The slightly worn molars of 



Fio-. 9. 



Morphippiis imbricatus : fifth right lower molar of adult, superior 

 aspect, uat. size. — Upper Cretaceous ; Pataguuia. 



Fis. 10. 



noi^^ 



(^^ 



Notohippus toxodontoides : fifth right lower molar, superior aspect, twice nat. 

 size. — Upper Eocene ; Patagonia, a. Crown of homologous tooth of 

 existing Equus caballus. 



the Cretaceous Toxodonts (Proadinotherium, Pronesodon) are com- 

 pletely similar to those of Morphippus :, so that it becomes almost 

 impossible to distinguish isolated molars of animals of the series 

 terminating with the Equidse from those belonging to animals of 

 the Toxodout line. 



Fis. 11. 



Archaophilus patrius: fifth right lower njolar, unworn (a) and worn [b), 

 superior aspect, four times nat. size. — Upper Cretaceous ; Patagonia. 



The teeth of Typotheria are a little different. Fig. 11 « repi'e- 



