90 THE NEOG-^IC REALM. [CHAP. 



described under the names of Perimys and Sphodromys, and thus 

 indicate that the family is essentially South American. The 

 largest known rodent is the extinct Megamys, from the Parana 

 beds ; the typical species of the genus being described as equal in 

 size to an ox. The porcupines (Hystricida), which form a 

 practically cosmopolitan family, sufficiently distinguished by their 

 spiny covering, are represented in South America by the two 

 arboreal genera Synetheres and Chcztomys, differing from all their 

 allies by their prehensile tails, although otherwise related to the 

 North American Erethizon, with which they form a separate sub- 

 family. Of Chcetomys an extinct representative occurs in the 

 cavern-deposits of Brazil; and in the Santa Cruz beds the family 

 is represented by apparently extinct generic types described under 

 the names of Stiromys, Acaremys, and Sciamys. Since fossil 

 remains of Hystrix date from the European Miocene or Oligocene, 

 there is distinct evidence of a connection between the early South 

 American rodents and those of the Old World ; while as Erethi- 

 zon is first known from the Plistocene of North America, it may 

 probably be regarded as a late immigrant into that country from 

 the south. 



The largest of all the families of the Hystricomorpha is that of 

 the Octodontidal, in which out of a total of nineteen genera 

 upwards of fifteen belong to the realm under consideration, while 

 the remaining four are African and mainly Ethiopian. In addition 

 to other features, the rodents of this family have the crowns of 

 the cheek-teeth marked by infoldings of enamel from both sides; 

 there are usually five toes to each foot ; and the general form is 

 more or less rat-like. Of the Neogaeic forms, the typical genus 

 Octodon is represented by the degu of Chili and Peru, which is a 

 large rat -like animal, with a brush-tipped tail; other species 

 occurring in Bolivia. The latter country is likewise the home of 

 the allied genus Habrocoma, the members of which vie with the 

 chinchillas in the delicate softness of their fur. Nearly related are 

 the burrowing South American tuco-tucos (Ctenomys\ characterised 



1 By some the family is divided into three; viz. Capromyidce, with the 

 West Indian Capromys, the S. American Myopotamus, and. the African 

 Triaulacodus ; Ctenodactylidcz, including the remaining African forms; and 

 Octodontidce, comprising all the other American types. 



