xii PHYLUM CHORDATA 541 



feature of the walruses is the presence of a pair of large tusks, 

 the enlarged canine teeth, projecting downwards from the 

 upper jaw. 



Though some of the rodents (beavers, water voles) are 

 aquatic, some (squirrels and tree-porcupines) are arboreal, 

 while others (the majority of the order) lead a terrestrial 

 life and are active burrowers ; they are on the whole a very 

 uniform group, and exhibit few such remarkable modifica- 

 tions as are to be observed in some of the other orders of 

 mammals. They are nearly all furry animals with five-toed, 

 plantigrade, or semi-plantigrade limbs. The tail is usually 

 elongated, and may be naked or covered with fur ■ but 

 sometimes, as in the rabbits and hares, it is very short. A 

 few special modifications, however, have to be noted in cer- 

 tain families of rodents. The flying squirrels have on each 

 side a fold of skin, the patagium, which serves as a parachute. 

 The African flying squirrels (Anomalurus) are remarkable 

 also on account of the presence of a series of overlapping 

 horny scales on the lower surface of the basal part of the tail. 

 The Jerboas (Dipus) and their allies are characterised by 

 the great relative length of the hind-limbs — the mode of 

 locomotion of these remarkable rodents being by a series of 

 leaps not unlike the mode of progression of the kangaroo — 

 and by the reduction of the number of the toes to three in 

 some of them. The porcupines {Hystricida) have numerous 

 elongated spines or " quills " among the hairs of the dorsal 

 surface, and some of them have prehensile tails. 



The Insectivora are, in general, small, furry, burrowing 

 mammals with plantigrade limbs and an elongated muzzle. 

 But there is a considerable range of modification within the 

 order in adaptation to different modes of life. The cobegos 

 ( Galeopithecits) have a fold of skin extending along each side 

 of the neck and body and continued between the hind-legs, 



