184 HIGH SCHOOL ZOOLOGY. 



on account of the ease with which they can be detached from the skin. 

 To the MuridcB belong the rats and mice (Mus), as well as the field-mice 

 {Arricola) and the aquatic musk-rat {Fiber). An interesting northern 

 genus is the lemming (Myodes), which often migrates in vast numbers 

 from one part to another in Northern Europe and Asia. The Dlpodidce 

 include the Egyptian Jerboa (Dipus), marked by the length of the hind 

 legs ; the same peculiarity is present but less developed in the American 

 jumping mouse (Zapus). In the Western prairies, two genera of pouched 

 gophers are met with, which constitute the family Geomyidce. They have 

 cheek -pouches which open on the cheeks outside the mouth, and are lined 

 with hair. As the gophers are burrowing forms, the fore feet are large 

 and armed with strong curved claws. The most truly aquatic of the 

 Rodents is undoubtedly the beaver {Castor fiber), the largest of our 

 species. As in the musk-rat, the hind toes are elongated and web- 

 bed, and the flat, scaly tail is very characteristic. The la^st family 

 — the SciurldcB — includes several genera, chiefly arboreal forms with soft 

 fur, and more numerous molars than the preceding, except the hares. 

 Among those which are distinctly terrestrial in their habits, may be 

 mentioned the gophers and prairie-dogs, while the chipmunk and the 

 woodchuck are intermediate, in this respect, between these and the true 

 arboreal squirrels and flying squirrels. The completely terrestrial forms 

 have cheek-pouches which open into the buccal cavity ; these are best 

 developed in the gopher ( SpermopMlus ) and the c\\v^iimn\i ( Tamias ), 

 while they are rudimentary in the prairie-dog (Cynomys), and absent in 

 the woodchuck (Arctomys) and the squirrel (Schirvs and Sciuropterus). 

 The last-mentioned genus includes the nocturnal flying squirrels, which 

 have a patagium stretching from the fore to the hind limbs and permit- 

 ting a slanting leap, such as we have already observed to be possible in 

 several mammalian orders. 



As representatives of tropical families, may be mentioned the Chin- 

 chilla of Chili and Peru, valued for its grayish fur, the Guinea-pig 

 {Cavia cobaya,now only known in the domesticated condition), the Capy- 

 bara (Hydrochoerus, the largest Rodent), the Paca (Coelogenys) and Agouti 

 {Dasyprocta) of Northern South America, which four genera have almost 

 hoof -like nails, and, like most of the Rodents, are gregarious in their 

 habits. y 



38. On the approach of cold weather many animals of differ- 

 ent classes pass through a lesting phase, which, in the warm- 

 blooded animals, is usually spoken of as hybernation. This 



