HISTORY OF THE MARSUPIALIA 627 



nearly all of the species are confined, North America having 

 but two or three. 



The more important American marsupials are given in the 

 table below : 



Suborder POLYPROTODONTA 



I. DiDELPHiiD^. Opossums. 



Didelphis, Opossum, Pleist. and Rec, N. and S. A. Chironedes, 



Water Opossum, Rec, S. A. f Peratherium, low. Eoc. to low. 



Oligo., N. A. '\Microbiotherium, Santa Cruz. \Eodidelphys, do. 



^Ideodidelphys, Casa Mayor. ^ Proteodiddphys, fCretaceous, S. A. 



II. THTLACYNiDiE. PredacBous Marsupials. 



^Cladosictis, Santa Cruz. ^Amphiprotdverra, do. ^Prothylaq/nus, 

 do. ^Borhycena, do. ^Proborhymna, Deseado. ■\Phxirsophorus, 

 do. ^ Prodadosictis, Casa Mayor. '\ Pseudodadosidis, do. 



Suborder DIPROTODONTA 



III. C^NOLESTID^. 



Cmnolestes, Rec, S. A. ^ Zygolestes, Parand. ^ Palceothentes, Santa 

 Cruz. ]Ahderiies, do. '\ Paloepanorthiis, Deseado. 



IV. tGrARZONIIDiB. 



'fGarzonia, Santa Cruz, f Halmarhiphus, do. f Cladodimis, do. 

 Suborder fALLOTHERIA 



V. fPl^GIAULACIDiE. 



tPoij/mostodore, up. Cretac andPaleoCjN. A. ^Ptilodus,do. ^Neo- 

 plagiaidax, Paleoc, N. A. 



VI. fPoi-YDOLOPID^. 



\ Propolymastodon, Casa Mayor. i[ Polydolops, do. ^Amphidolops, 

 do. 



Despite all their diversity of appearance and habits, the 

 unity of structure among the marsupials is such that the forma- 

 tion of groups of higher than family rank is very difficult, and 

 it is by no means certain that the suborders currently accepted 

 correspond to the facts of actual relationship. 



Except in certain extinct South American genera, there is 

 very Uttle change of teeth, only the last premolar in each jaw 

 being replaced. Sometimes the temporary tooth is long 

 retained in function and, more rarely, it is shed very early ; 

 while in several genera no replacement of teeth has been ob- 



