242 



KNOWLEDGE. 



[No\-EMBER 1, 1894. 



dormice {Mijoxidae), the jumping mice and jerboas 

 {Dipodid(e), the whole great mouse-tribe (Murula-), a.s 

 represented by mice, rats, voles, hamsters, and their kin ; 

 the mole-rats' (Spalacidre), and the American pouched 



Fig-. 1. 



-Upper Surface of the Lower Jaw of the Coypu, to 

 chisel-like front teeth and the grinding molars. 



rats {Geomyidce). Of these, the only one represented in 

 South America is the cosmopolitan mouse family. In this 

 group, however, the true rats and mice of the Old World 

 are totally wanting (except through involuntary intro- 

 duction by man) as indeed they are throughout the New 

 World ; their place being taken by the white-footed mice, 

 common both to North and South America, and nearly 

 allied to the hamsters. In addition to these, there are a few 

 genera belonging to the family which are quite peculiar to 

 South America, one of the best known of these being the 

 groove-toothed mice {Fhithrodon), a representative of which 

 extends as far south as Tierra del Fuego. 



By this time the reader wUl probably begin to think 

 that, instead of South America being the home of the 

 rodents, it is precisely^ the country where those animals 

 are rarest, seeing that out of the nine families noticed 

 above only two are represented there at all, and both of 

 these somewhat poorly. His opinion will, however, at 

 once change when we have considered the third great group 

 of the order, which may be known as the porcupine-like 

 rodents. The members of this group are readily dis- 

 tinguished from those of both the preceding sections by 

 the structure of the lower jaw, although this difference 

 can, of course, only be seen in the skeleton. Whereas in 



both the squirrel-like and mouse-like sections the hinder 

 lower projection of the lower jaw, technically known as 

 the angle, takes its origin from the inferior edge of the 

 socket for the lower chisel-Uke teeth, m the present group 

 the same projection arises from a prominent ridge 

 running along the side of the jaw itself, as shown 

 in Fig. 1. Of the six families into which the 

 porcupine-like rodents are di%nded, four are ex- 

 clusively confined to South America, while the fifth 

 (OctdJi intidcr) is ma.m\j South American and West 

 Indian, although with a few representatives in 

 Africa south of the Sahara desert. On the other 

 hand, the porcupine family {Hystricidce) is almost 

 cosmopolitan, although the American representatives 

 are so distinct from their Old World allies as to form 

 a separate sub-family ; while of the three genera 

 constituting the latter, one is North American, and 

 the others are mainly South American, although one 

 has a single species ranging as far north as Mexico. 

 Of the last group of the order, namely, the hare- 

 like rodents, as represented by the hares and rabbits 

 (Lcporidie) and the picas iLaijomi/ida'), we have but 

 little to say, seeing that its occurrence in South 

 America is limited to two species of hares. 



It will thus be seen that, out of a total of seven- 

 teen families, nine are represented in South America ; 

 and of these nine, four are absolutely peculiar to 

 that country and the adjacent islands and isthmus, 

 while a fifth has representatives elsewhere only in 

 Africa, and a sixth (the porcupines) has two genera 

 which are practically only South American, and 

 are distinguished from all their allies by their 

 prehensile tails. The significance of these remark- 

 able facts in geographical distribution will be made 

 apparent when we state that, of the other great 

 zoological regions into which the globe has been 

 mapped out, there are only two which have any 

 families of rodents peculiar to them, the maximum 

 number of such peculiar families being two. Thus 

 Africa south of the Sahara, constituting the 

 lilthiopian region of zoologists, has the African flying 

 squirrels, which are nearly allied to the true 

 squirrels, and are represented only by a single 

 genus, with a comparatively small number of species ; 

 while North America, or the Nearctic region, 

 has the sewellels, with one genus, and the pouched rats, 

 with five. 



Before entering on the significance of these facts, we 

 must devote a short space to the consideration of some of 

 the more remarkable of the South American types of 

 porcupine-like rodents, by which aloue the reader will be 

 enabled to appreciate their peculiarities and numbers. 

 Commencing with those families which are confined to the 

 Neotropical region, as zoologists term South America, 

 together with Central America and the West Indies, we 

 have first of all the caries {Cai-iidce), which are heavily- 

 built rodents, with four front and three hind toes, rudi- 

 mental or short tails, and the molars divided by transverse 

 folds of enamel into a number of thin plates lying parallel 

 to one another, after the fashion shown in Fig. 2. 

 The typical representatives of the family are the true 

 cavies (Curia), of which the guinea-pig is a domesticated 

 descendant, having assumed a coloration quite different 

 from the uniform oUve-brown tint characteristic of its wild 

 ancestors. Quizos, as these animals are called in the 

 Argentine, may be found not only among aquatic plants 

 ■in marshy districts, and skulking in the tufts of coarse 

 grass on the pampas, but in the neighbourhood of human 

 habitations will not unfrequently take up their residence 



ihovr the 



