350 BRANCH CHORDATA 



Old World camels, as well as giraffe-like forms. " That the 

 camel got the pads on his feet, the water-pockets in his stomach, 

 and the other drought and sand resisting arrangements from an 

 ancestor that began in the United States a million or more years 

 years ago " has been proved by Cope, Doctor Wirtman, and his 

 assistants. The oldest fossil deer types are Miocene. They 

 were small, hornless creatures. The first horned deer were in 

 the middle Miocene, when the horns were bifid. The giraffe, 

 or its close allies, existed in the Miocene. The bison existed in 

 the Miocene, as shown in fossil Bos sivalensis from India. The 

 Pliocene life included a variety of oxen and two North American 

 bisons. 



Order VII. Rodentia or Gli'res. — Rodents are exceedingly 

 numerous and well-known mammals, covered with fur or spines. 

 They are generally small, varying in size from the porcupine, of 

 about 3 feet in length, to some of our small mice, not over 4 or 5 

 inches long. 



The one distinguishing characteristic of rodents is their teeth. 

 (See Fig. 288, p. 355.) These have enamel on their front surfaces 

 only. Thus, their incisors, which grow continuously from per- 

 sistant pulps, are always chisel shaped and kept sharp by the 

 wearing away of the posterior surfaces. There are never more 

 than two incisor teeth in the lower jaw, and only two in the 

 upper, except in Lepor'idce, where there are four. The lack of 

 canine teeth in all rodents leaves a space or diastema between 

 the incisors and the molars. The molars vary in number from 

 two to six on each side of each jaw. There is a hairy ingrowth 

 in the mouth back of the incisors, which serves to catch the 

 particles when the animals are gnawing. They are usually 

 vegetable feeders, but some arc carnivorous or omnivorous. 

 The intestine is long and the cecum large (except in dormice) and 

 often complicated. 



As a rule the clavicle is present. Most rodents are five toed. 

 The toes have claw-like nails. They arc usually plantigrade or 

 semiplantigrade. Thc^ cerebrum is small and nearly or quite 

 smooth, but in some of the larger forms (the beaver excepted) 

 it is well convoluted. The cars and eyes are well developed. 

 The voice is a squeak or squcnil. Their defense is by spines, 

 as in the porcupine; by biting, as in the rat; and by flight or 



