MAMMALS THAT GNAW. 



89 



mouth. As a rule collar-bones are well developed, although they may 

 be wanting. There are generally five toes to the fore-feet, but m tlie 

 hind-feet the number may be reduced to four, or even three ; the claws 

 being usually sharp and curved. A single species of Spermoiihilus has a, 

 ringed tail. 



The order has a cosmopolitan distribution, being fairly well represented 

 even in Australia, New Guinea, and Madagascar ; but it is in South America 

 that it attains its maximum development, tlie number of family types peculiar 

 to that region being very large. In size. Rodents vary from that of a rather 

 small pig to that of the smallest shrew, the harvest-mouse being one of the 

 most minute of Mammals. The carpincho (Hydrocharus capivara) of South 

 America is the largest of the Rodents. As regards habits, all feed almost ex- 

 clusively upon vegetable substances 

 (except perhaps one rat from the 

 Philippines, and a second from South 

 America), but in other respects they 

 present great diversity. The flying- 

 squirrels, for instance, are flying, 

 arboreal, and nocturnal, whereas 

 the ground-squirrels are terrestrial. 

 Others again, such as the marmots, 

 form large colonies, the members of 

 which live in burrows, and are to a 

 great extent diurnal ; while others, 

 like the beaver and coypu, have 

 taken to an aquatic life. In struc- 

 ture, Rodents are so much alike that 

 somewhat obscure osteological characters have to be relied upon in order to 

 divide them into groups. 



Agreeing with the great bulk of the order in the possession of only a single 

 pair of upper incisor teeth, the members of this and the three following 

 families constitute a group (Sciuromorpha) chiefly charac- 

 terised by certain peculiarities in the structure of the skull. African Flying- 

 In this portion of the skull the cheek or zygomatic arch Squirrels. — 

 (that is to say the long slender bar of bone running along Family 



the lower border of the socket of the eye) is chiefly formed Ano^naluridce. 

 by the bone known as the jugal, which is not supported by 

 a backwardly-directed process from the upper jawbone, or maxilla ; postor- 

 bital processes arising from the frontals to define the hinder margin of the 

 sockets of the eyes may or may not be developed ; and in the lower jaw 

 the hinder, or angular portion arises from the socket of the lower incisor 

 tooth. 



Although flying-squirrels are abundant in the Oriental countries, in Africa 

 south of the Sahara their place is taken by a separate family, comprising t^^■o 

 distinct genera. These African flying-squirrels difier from their Asiatic 

 cousins in that the parachute, by means of which they take their long flying 

 leaps, is supported in front by a rod of cartilage projecting from the elbow, 

 instead of from the wrist ; and an additional peculiarity is the presence of a 

 row of overlapping horny scales on the under surface of the tail, which are 

 believed to be of use in climbing. The typical representatives of the family 

 are the short-tailed flying-squirrels {Anomalurus), most of the species in- 

 habiting West Africa, although one is found in Equatoria, and a second near 



Fig. 64.— TirE Carpincho 

 {Kydrochoirus capivara). 



