296 TEKTEBRATE AW TWALS 



The last family of the Cetacea is that of the DdphinidcB, 

 comprising the Dolphins (Fig. 146) and Porpoises. "Diey have 

 numerous conical teeth in both jaws, and the nostrils open by 

 a single aperture on the top of the head. The Dolphins are 



Pig. 146. — Cetacea. Tlie Conunon Dolphin {Delphinua delphis). 



iohabitants of the sea, but two species live in rivers — one in 

 India, and the other in America. The Porpoises are also 

 marine, and occur in all seas. The most remarkable of the 

 DeljahinidcB is the Narwhal or Sea-Unicorn, which is found in 

 the Arctic seas, and which attains a length of as much as fif- 

 teen feet in the body alone. The chief pecuHarity of the Nar- 

 whal is iu the dentition. The females, as a rule, have no teeth, 

 the upper jaw alone having two rudimentary incisors which 

 never cut the gum. In the males, however, while the lower 

 jaw ie without teeth, one of the two central incisors of the 

 upper jaw 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 fucction of this extraordinary tooth is doubtless offensive. 



Okbee VI. Unguxata (Lat. ungula, a hoof). — This order 

 is often spoken of as that of the Hoofed Quadrupeds, and is 

 one of the largest and most important of the orders of Mam- 

 malia. The order is characterized by having all the four limbs 

 and by having that portion of the toe which touclies 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 sub- 

 stances. 



The Ungulata are divided into two great primary sections, 

 according as the toes are even or odd in number : 



