182 



LAND MAMMALS IN THE WESTERN HEMISPHERE 



and beavers all being lacking. In the same way the rat and 

 mouse division is represented by a single family. The vesper 

 or white-footed mice (Sitomys) have invaded the southern 

 continent and a number of peculiar genera have arisen there, but 

 all of northern ancestry, such as the groove-toothed mice 

 (Rheithrodon) and the fish-eating rats (Ichthyomys). The 



Fig. 103. — Brazilian Tree Porcupine (Coeiidou prehensilis). — By permission of the 



N.Y. Zoolog. Soo. 



voles, or meadow-mice, the muskrats, jumping mice, kan- 

 garoo-rats and pocket-gophers of the northern continent are 

 all absent. While the immigrant suborders have thus but 

 one family each in South America, the case is very different 

 with the fourth or porcupine group, of which that continent 

 is to-day, as it has been for ages past, the headquarters. No 

 less than six families and twenty-nine genera are known, all 

 of the genera and four of the families being restricted to the 

 Neotropical region. Contrast this assemblage with the ex- 

 treme scantiness of this group in North America, where but 

 a single genus, the Short-tailed or Canada Porcupine (Ere- 



