190 STRUCTURAL AND SYSTEMATIC ZOOLOGY 



retractile ; the ears are small, and tail short. Bears, bad- 

 gers, and raccoons are well-known examples, c. Digiti- 

 grades keep the heel raised above the ground, walking 

 on the toes. The majority have long tails. Such are 

 the weasels, otters, civets, hyenas, foxes, jackals, wolves, 

 dogs, cats, panthers, leopards, tigers, and lions. . The 

 last five differ from all others in having retractile 

 claws, and the radius rotating freely on the ulna. The 

 cats have thirty teeth ; the dogs, forty-two, or twelve 

 more molars. In the former, the tongue is prickly ; in 

 the latter, smooth. 



6. Rodentia, or gnawers, are characterized by two 

 long, curved incisors in each jaw, enameled in front, 



FIG. 180. Skull of a rodent (Capybara) : 22, premaxillary; 21, maxillary; 26, molar; 

 27, squamosal; 73, lachrymal; 15, nasal; n, frontal; 4, occipital processes, un- 

 usually developed; z, incisors; a, angle of lower jaw. 



and perpetually growing ; they are specially formed for 

 nibbling. Separated from them by a wide space (for 

 canines are wanting), are the flat molars, admirably 

 fitted for grinding. The lower jaw has longitudinal 

 condyles, which work freely backward and forward in 

 longitudinal furrows. Nearly all have clavicles, and 

 the toes are clawed. The cerebrum is nearly or quite 

 smooth, and covers but a small part of the cerebellum. 

 All are vegetarian. 



More than one half of all known mammals are rodents. 



