HISTORY OF THE ARTIODACTYLA 



369 



the lower Miocene, there is very little change except in size, 

 all the essential features of structure remaining the same • 

 the genera are therefore distinguished by modifications of very 

 secondary importance, and it is a question whether all the 

 species should not be included in a single genus. The European 

 genus ^Entelodon, which gives its name to the family, is so like 

 the American forms that by most writers the White River 

 species are referred to it. It is of interest to note that the 

 tgiant pigs have also been found in the marine Miocene of 

 New Jersey, one of the few records of the Tertiary land mam- 

 mals of the Atlantic seaboard. 



At present, the jentelodonts proper cannot be traced back 

 of the lower White River beds, nor are they found in any more 

 ancient formations in Europe. It is, therefore, probable that 

 they were immigrants in both of these continents, presumably 

 from Asia. 



The whole Eocene of North America had a series of pig-like 

 animals, called the tachsenodonts or fshort-faced pigs, which 

 seem to have 

 been related to 

 the fentelo- 

 donts. They 

 ended their 

 career in the 

 Uinta just be- 

 fore the appear- 

 ance of the fen- 

 telodonts, and 

 it would be nat- 

 ural to suppose 



that thp lattpr ■^''^' ^^^" — Skull of tshort-faced pig {\Achcenodon robustus) 

 from the Bridger Eocene. Princeton University Museum. 



were descended 



from them. If, however, the principle that an organ or 

 structure once lost can never be regained, is valid, then there 

 can be no relation of ancestor and descendant between the 



2b 



