1899.] 



PLEXODOXT MOL.VllS OF MAM:M.VLS. 



5G3 



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

 Ungulates, e.ij. the Toxodontia and Typofheria, 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 



Fig. 9. 



MorpMppus imhricafus: iifth right lower molar of atlalt, superior 

 aspect, nat. size. — Upper Cretaceous ; Patagouia. 



Fig. 10. 



^yrvu 



<^^ 



oyiA^ 



Notokippiis toxodonfoidcs : ilfth right lower inolar, superior aspect, twice nat. 

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

 existing Equus cahaUus. 



the Cretaceous Toxodonts (ProadinotJterimn, Pronesodon) ai*e com- 

 pletely similar to those of Moi'phijjpus ; 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. 



Fig. 11. 



*yruL^ 



ArchiBophilus patriiis : fifth right lower molar, unworn (a) and worn (i), 

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



The teeth of Typotheria are a little different. Fig. 11a repre- 



