PART III. GLIRES. 



The rodent fauna of the Santa Cruz beds consists exclusively of Hystri- 

 comorpha, a very marked contrast to the modern fauna of the same 

 region, in which the Myomorpha predominate. All of the non-hystri- 

 comorph rodents which now inhabit South America are of northern 

 origin, their ancestors having reached South America not earlier than the 

 close of the Miocene, when the great intermigration of the mammals of 

 the two continents began. 



Confining our attention to the Hystricomorpha, we find that the Santa 

 Cruz rodents are very closely allied to recent South American genera and 

 are all referable to existing families and even, with one exception, to 

 existing subfamilies. The richness and variety of this assemblage are 

 surprising, for, except the Dasyproctidce and Dinomyidce, all the recent 

 families are represented and the number of genera is comparable to that 

 which is spread over the entire Neotropical region of to-day. It must not 

 be supposed that the full number of Santa Cruz genera has been already 

 discovered, though it is improbable that the list will be very greatly 

 extended in the future ; for, from the large collections made by Messrs. 

 Hatcher & Peterson for Princeton University and by Mr. Brown for the 

 American Museum of Natural History, not a single new genus has been 

 added to those described by Ameghino, and even of new species the 

 number is very small. 



The general aspect of the Santa Cruz rodents is strikingly modern and 

 a hasty examination might easily lead to the conclusion that they were 

 essentially the same as the recent forms, but such a conclusion would be 

 quite erroneous. The genera are all extinct and a considerable number 

 of them have left no successors ; these latter are, for the most part, the 

 more or less modified survivors of earlier faunas and, for that reason, are 

 often of great phylogenetic importance. 



The subjoined table, which follows Thomas' arrangement of families 

 and subfamilies ('96, 1015 ff.), will serve to give an idea of the compara- 



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