562 



ZOOLOGY. 



they are sometimes called Cavicorns. In most horned 

 mammals, the horns are persistent ; in the deer they are 

 dropped annually ; in the prong-horned antelope (Fig. 487) 

 the horns are also shed annually. 



The mammary glands are modifications of the tegument- 

 ary glands which are found in all vertebrates except fishes. 

 In the duckbill and spiny ant-eater (Echidna), these glands 

 retain their simple elementary nature. In all others nip- 

 ples are developed (Fig. 486). They correspond in general 

 to the number of young in a litter. 



The dentition needs careful study in connection with the 



Fig. 488. Skull of Ant-eater. After Owen. 



Pig. 487. -Hollow 

 horn of the Prong 

 horned Antelope. 



Fig. 489. Skull of a Porcupine. After Owen. 



fossil remains of mammals, as the different orders are char- 

 acterized in great part by the differences in the form and 

 number of the teeth, which are intimately correlated with 

 the structure of the digestive organs and the nature of the 

 limbs ; thus while vertebrae are useful in identifying or re- 

 storing fossil reptiles, the teeth are especially serviceable in 

 classifying fossil mammals. Some existing forms are en- 

 tirely toothless, as the duckbill, where the teeth are repre- 

 sented by horny plates, and the ant-eater (Fig. 488). While 

 the sloths have no incisors, these are present and very 

 large in the rodents, but the canines are absent (Fig. 489), 



