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 hornsare also shed annually. The giraffe’s hornsare hairy. 
The mammary glands are moditications of the tegument- 
ary glands which are found in all vertebrates except fishes. 
In the duckbill and spiny ant-eater (Zchidna), 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. 487. — Hollow 
horn of the Prong 
horned Antelope, 
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 vertebre 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). 
