MAMMALIA 



131 



number of forms which, though they differ greatly among them- 

 selves in appearance, all possess large hair-lined cheek-pouches. 

 The Common Pocket-Gopher {Geomys bursarius) looks some- 

 thing like a compromise between a rat and a mole. It is a 

 burrowing form inhabiting the central plains watered by the 

 Mississippi and its tributaries. The Common Kangaroo -Rat 

 {Dipodomys Phillipsi) is a desert animal with long hind-limbs 

 and tail (fig. 90), enabling it to adopt the mode of progression 

 characteristic of the kangaroo. The 

 Banded Vock.&\.'yioM%&{Perognathus 

 fasciatus) of the States resembles 

 the last-named form on a small scale. 



(4) The Jumping- Mouse or 

 Jerboa Family includes species 

 which, like the Kangaroo- Rat and 

 Pocket-Mouse of the last family, are 

 specialized in relation to a springing 

 mode of progression. They are, 

 however, devoid of cheek-pouches. 

 The species are chiefly found in 

 Africa and Asia, though the family 

 is also represented in South Europe 

 and North America. A good illus- 

 trative species is the Siberian Jerboa 

 {Alactaga decumana) (fig. 91). 



(5) The Dormouse Family embraces small mouse-like animals 

 which have adopted an arboreal life, and in some respects 

 have become specialized in much the same way as squirrels. 

 Species are found in Europe, Asia, and Africa, and there is 

 one British form, the Common Dormouse {Muscardinus avel- 

 lanarius), abundant in the south of England, but absent from 

 Ireland and the northern part of Great Britain (fig. 92). 



4. The Porcupine group of Rodents takes in a large 

 number of forms, of which one common character is the separate- 

 ness of the two bones (tibia and fibula) of the lower leg. The 

 group is predominatingly American, and specially characteristic 

 of South America. The following five families are included: — 

 (i) Octodons; (2) Porcupines; (3) Chinchillas; (4) Agoutis; and 

 (5) Cavies. 



(i) Octodons are somewhat rat-like forms mostly found in 



Fig gi — The Jerboa {Alactaga decumana) 



