ORDEKS OF MAMMALIA. 331 



the Arctic seas, and which attains a length of as much as fifteen feet 

 in the body alone. The chief peculiarity of the Narwhal is in the 

 dentition. The females, ;is a rule, have no front teeth, the upper 

 jaw alone having two rudimentary canines which never cut the gum. 

 In the males, however, whilst the lower jaw is without teeth, one 

 of the two canines of the upper jaw (the left one) is enormously 

 developed, and grows throughout the life of the animal. It forms 

 a tusk of from eight to ten feet in length, the whole surface of 

 which is spirally twisted. The function of this extraordinary tooth 

 is doubtless offensive. 



Order VI. Ungdlata. 



This order is often spoken of as that of the Hoofed Quadrupeds, 

 and is one of the largest and most imjjortant of the orders of Mam- 

 malia. The order is characterised by having all the four limbs, and 

 by having that portion of the toe which touches the ground encased 

 in a greatly expanded nail or hoof. There are never more than four 

 full-sized toes to each leg, and owing to the presence of hoofs the 

 limbs are useless for grasping, and are only of use in locomotion and 

 in supporting the weight of the body. There are always two sets 

 of teeth, and the molars have broad crowns adapted for grinding 

 vegetable substances. 



The Ungulata are divided into two great primary sections, accord- 

 ing as the toes are even or odd in number : — 



A. Perissodactyla, or Odd-toed Ungulates, in which the toes are 

 odd in number — either one or three.^ If horns are present, they are 

 not in pairs (in any living form). 



B. Artiodactyla, or Even-toed Ungulates, in which the toes are even 

 in number — either two or four; and if horns are present, they are 

 in pairs. 



Section A. Perissodactyla. — The livijig Perissodactyle Ungulates 

 are the Rhinoceros, the Tapirs, and the Horse and its allies. The 

 Rhinoceroses are extremely large and bulky brutes, having a very 

 thick and nearly hairless skin, usually thrown into deep folds. The 

 feet (fig. 237, B) are furnished with three toes each, all encased in hoofs. 

 The nose (fig. 238) is furnished with one or two horns, composed of 

 longitudinal fibres compacted together, and not having any central 

 core of bone. When there is only one horn, it is, of course, unsym- 

 metrical ; and when there are two, these are not paired, but (except 



1 The fore-feet of tlie Tapirs are even-toed, but the hiud-feet are perissodac- 

 tyle ; and in some extinct types we find five toes to the foot. 



