740 Comparative Morphology of Chordates 



number of digits. The digits bear claws, in some cases so blunt as to 

 resemble small hoofs. 



Examples: Mouse, rat (Mus); squirrel (Sciurus — dental for- 



mula, i 



10 2 3\ 



j, Ct, py, m ~ 1, gopher, woodchuck, beaver (Castor), 



guinea pig (Cavia), chinchilla, porcupine whose spines resemble 

 those of the hedgehog (an insectivore). The largest living rodent is the 

 South American capybara, semiaquatic, about 4 feet long, resembling 

 a gigantic guinea pig: (Figs. 546, 549, 550). 



Fig. 548. Skull of the porcupine, (v) Antorbital 

 vacuity; (11) frontal; (22) premaxillary. Notice the 

 relatively great size of the premaxillaries, which carry 

 the incisor teeth. (Courtesy, Owen: "Comparative 

 Anatomy and Physiology of Vertebrates," London, 

 Longmans, Green & Co., Ltd.) 



The Order Rodentia includes more living species than any other 

 mammalian order, some estimates of the number approaching 3000. 

 Mostly terrestrial, some are burrowing, some arboreal, a few are 

 aquatic or semiaquatic, and "flying squirrels" sail by spreading out a 

 pair of broad folds of skin extending between fore- and hindlegs. 



Distribution: World-wide; especially numerous in South America, 

 relatively few in Australia and Madagascar. Rabbits are now abundant 

 in Australia but are not native there, having been introduced by man. 



Order 8: LAGOMORPHA. Two pairs of upper incisors, those of 

 the more lateral pair being very small ; one pair of lower incisors. Tail 



.203 3 

 vestigial. Dental formula for rabbit, i y, c t, p ^, m ~ This Order in- 

 cludes only the rabbit, hare, and pika (Fig. 551). 



Older classifications usually include the lagomorphs as a Suborder 



