390 CLASSIFICATION OF VERTEBRATES. 



SCIURID^E ; molars f or |, fore feet four-toed, hind pentadactyl ; tail 

 covered with fur. To the family belong the woodchucks (Arctomys}, 

 prairie dogs (Cynomys}, gophers (Spermophilus} , chipmunks (Tamms}, 

 squirrels (Sa'urus), and flying-squirrels (Pteromys and Sciuropterus) , the 

 latter sailing, rather than flying, through the air by means of an inter- 

 membral membrane on either side of the body. Sciurus appears in the 

 eocene. The CASTORID^E, or beavers, the habits of which are so well 

 known, have the molars , the feet webbed, and the tail flattened and 

 scaly. Castor fiber, the beaver, formerly ranged over the northern parts of 

 both continents, but has been greatly restricted. The genus dates from 

 the pliocene ; the allied Stenofiber is miocene. The small families HAPLO 

 DONTIDJE and ANOMALURID.E are represented by Haplodon, the sewellel 

 of Oregon, and Anomalurus, a flying squirrel-like form from Africa. The 

 fossil family, ISCHIROMYID^E, occurs in the eocene and miocene of North 

 America. 



SUB-ORDER 2. MYOMORPHA. 



Incisors ^, molars f or f ; clavicle usually present; tibia and fibula 

 fused. The DIPODID^E, or jumping mice, including Zapus of the United 

 States, Dipus, the jerboas of Europe, and Pedetes of South Africa, have 

 the hind legs long, the toes being 5, 3, and 4 respectively, in the three 

 genera. A much larger family is the MURID^E, in which there are no pre- 

 molars, the molars are f to f , and the tail generally naked and scaly. Over 

 three hundred living species are known. Cricetus, including the hamsters 

 of the old world, and Hesperomys, the white-footed mice of the new, have 

 the molars f . In Arvicola and its allies the tail is round, and the molars 

 rootless. These are commonly known as field-mice or voles. The migra- 

 tory lemmings of northern Europe belong to Myodes. Fiber includes our 

 muskrat. In Mns, which contains our mouse (M. Hiusculus*), and our rats 

 (M. decumanus, the brown rat, and M. rattns, the black rat, the latter 

 driven out by the former), the incisors are narrower and the molars rooted. 

 The family dates back to the later pliocene. The MYOXID^E of Europe, 

 represented to-day by the seven-sleeper, Myoxus gtis, dates from the eocene. 

 The GEOMYID^:, or pocket gophers, receive their name from the enormous 

 cheek pouches. The legs are fitted for burrowing, and the molars Geo- 

 mys and Thomomys occur in our central region. Farther west is Sacco- 

 mys with much more delicate skull. BATHYERGID/E : Spalax, the blind 

 mole-rat of southeastern Europe, and Bathyergus, the strand-rat of South 

 Africa. Lophiomys, a peculiar arboreal rat with hairy tail, from north- 

 eastern Africa, is nearest to Cricetus. 



SUB-ORDER 3. HYSTRTCOMORPHA. 



Skull with very large infraorbital canal ; teeth / \, c , / \, m f ; zygo- 



matic process large, clavicles perfect or imperfect ; tibia and fibula separate. 



The hystricomorphs appear in the eocene of Europe and South Amer- 



