88 THE NEOG^IC REALM. [CHAP. 



Muridce, however, as in North America, the true rats and mice 

 (Mus) of the Old World are entirely wanting, except through 

 human introduction ; their place being taken by the white-footed 

 mice (Sttomys) common to the whole of the New World, and 

 nearly related to the European hamsters. Being essentially a 

 northern type, they are certainly recent immigrants into South 

 America from the north ; and the same is doubtless true of certain 

 genera of the family now peculiar to this realm, such as the fish- 

 eating rats (Ichthyomys\ of Peru and Venezuela, the groove-toothed 

 mice (Rhithrodori), one of which ranges as far south as Tierra del 

 Fuego, and the Brazilian genus Holochilus, which is another form 

 allied to the hamsters. Although the cotton-rat (Sigmodon) occurs 

 in Central America, and occasionally wanders still further south, 

 the whole tribe of voles and their kindred are absent from the 

 entire realm. 



Represented by six families, all of which occur within its limits, 

 the porcupine-like group, or Hystricomorpha, may be regarded as 

 the characteristic rodents of Neogsea. From both the preceding 

 sections of the order the members of this section may be readily 

 distinguished by the angle (or hinder inferior projection) of the 

 lower jaw taking its origin from a prominent ridge running along 

 the side of the jaw, instead of from the inferior edge of the socket 

 for the incisor teeth. Of the families confined to this realm, that 

 of the cavies (Caviidcz], includes heavily-built rodents, with four 

 front and three hind toes, rudimental or short tails, and the 

 cheek-teeth divided by transverse folds of enamel into a number 

 of thin parallel plates. The genera of this family include the true 

 cavies (Cavia] so well known through the domestic guinea-pig 

 all of which are small short-limbed forms ; the larger and taller 

 Patagonian cavy (Dolichotis}; and the carpincho, or capivara 

 (Hydrocha>rus\ which is the largest living member of the order, 

 and is characterised by the large number of plates going to form 

 the last molar tooth in each jaw. Although they do not appear to 

 have been recorded from the Santa Cruz beds, remains of members 

 of this family occur in the Parana 1 horizon, and also in that of 



1 There is some confusion with regard to the age of the Parana beds. They 

 are overlain by marine strata which have been identified with the Patago- 

 nian ; but it is more probable that they are newer than the Santa Cruz beds, 



