308 PRACTICAL LESSONS IN SCIENCE. 



and perhaps lessens specific gravity. This group includes the 

 Dolphins, Whales and Sirenia. 



The dolphin is carnivorous, having both jaws armed with con- 

 ical teeth. They live on fish, and frequent the mouths of large 

 rivers. The narwhals have two upper teeth, which in the female 

 are small, but in the male the one on the left side usually be- 

 comes a spiral tusk, which sometimes attains a length of twenty 

 feet. The sperm whale has an enormous head, nearly one-third 

 the length of the body. The head is swollen by the accumulation 

 of fat spermaceti. They inhabit the colder regions, and live 

 mainly on cephalopoda. The whalebone whales have large 

 heads and wide mouths without teeth. From the palate and 

 upper jaw rows of whalebone plates project downward, forming 

 a kind of sieve which strains the small medusae and other forms 

 of life from the water as it flows out. The sirenia have a dis- 

 tinct neck and no canine teeth, they feed on fuci and seaweed 

 along the coast. They are known as sea cows, or manatees, on 

 the coasts of America, and as dugongs on the coasts of the In- 

 dian ocean. Stellar 's sea cow of the north Pacific has become 

 extinct within the last 200 years. 



The Perissodactyla are large herbivorous animals having one 

 or three hoofs, and in all, the teeth are well developed. The teeth 

 are divided into incisors, canines, premolars and molars. In 

 this group the premolars and molars form a continuous series of 

 broad grinding teeth all much alike. The dorsal-lumbar verte- 

 brae are usually 23, never less than 22. The middle digit or toe 

 is always largest and symmetrical in form. 



The Tapirs are short-haired animals of medium size, having 

 four hoofs on the anterior legs and three on the posterior. There 

 are 6 incisors, 2 canines and 6 molars in each jaw, while there are 

 8 premolars in the upper and 6 in the lower jaw. They have a 

 short proboscis, eat leaves and young twigs, and live in South 

 America and Southeastern Asia. 



The Rhinocerotidse are large, unwieldy, thick skinned animals 

 bearing one or two epidermal horns on the nasal bones. In 

 each jaw there are 4 rudimentary incisors, 8 premolars and 6 



