xxv ] CHEIROPTERA 705 



level of the water so as to cover the entrance to their burrows. By 

 this means large tracts of country have been converted into swamp. 

 The Porcupines (HYSTRICIDAE) have some of their hairs developed into 

 sharp spines which make them awkward objects to handle. In the 

 Canadian Porcupine, Erethizon dorsatus, the spines are concealed by 

 the fur. The commonest Ground-squirrel of North America is the 

 Chipmunk, Tamias, an active little animal with large eyes and a short 

 hairy tail. The Prairie Marmot, Cynomys, the so-called Prairie-dog or 

 Ground-hog is also a Ground-squirrel with a very short tail. It lives 

 in communities, burrowing in the ground and its home is often shared 

 by a small burrowing Owl, Athene cunicularia, and by a Rattlesnake, 

 which probably eats the young Marmots. The Musquash or Musk-rat, 

 Fiber zibethicus, one of the MURIDAE, is peculiar to North America, 

 and very widely distributed in suitable places (Fig. 355). It is 

 aquatic, living on roots and water-plants and is most active at night. 

 It constructs burrows in the banks of streams, the openings of which 

 are under water. Its fur is valuable. 



Order IX. Cheiroptera. 



The Cheiroptera (Gr. x/>, a hand ; vrcpov, wing), or Bats, have 

 not in their general organisation, in teeth or brain or stomach, 

 departed far from the Insectivora; their great distinguishing 

 feature is the modification of the arm into a wing. As in Birds, the 

 fore-arm is bent up on the upper arm, the wrist bent down on the 

 fore-arm ; but unlike Birds' wings the flying membrane is of skin, the 

 greater part of which is stretched between the fingers of the five- 

 fingered hand, only the smaller part extending, as in Birds, between 

 the elbow and the side of the body. The hand is enormous, the 

 little finger being, as a rule, very greatly developed and as long as 

 the rest, while the thumb alone is small and is not included in the 

 membrane but ends in a hook-like nail (Fig. 356). Part of the 

 membrane extends down the thighs, and in some even the tail is 

 involved. The knees are turned outwards and backwards, a most 

 extraordinary position which would mean dislocation if the hip-joint 

 of any other Mammal were forced into it but which is rendered 

 possible in Bats owing to the fact that in them the pubes are not 

 directed inwards so as to meet one another in a symphysis but slope 

 outwards and are consequently widely separated from one another 

 (Fig. 356). When the Bat crawls it hooks itself along with its 

 thumb-nail and pushes itself awkwardly with its hind feet. It has 

 &. & M. 45 



