578 CHORD ATA. 



but these gradually arise as small protuberances covered with hair or 

 "velvet." When the antlers are full-grown the "velvet" is rubbed off 

 by the deer by friction against trees or other objects until the bony 

 antler alone remains. The branches of the antler are called "tynes," 

 and in those species with many tynes the number of these increases every 

 year. Antlers are usually confined to the male sex. 



The musk-deer {Moschiis) and the water-deer {Hydropotes) have no 

 antlers in either sex, but, on the other hand, they retain the upper canine 

 teeth as long sharp tusks. 



In all the PecoJ'a the placenta is cotyledonary, a specialised derivative 

 of the diffuse. 



The true deer are not found in the Ethiopian region, their place 

 being taken by the "horned" antelopes. To this region are confined 

 the giraffes. The sheep, oxen and goats are more or less northern 

 forms, the north temperate regions of Eurasia and N. America being 

 their headquarters. 



The above five families of Artiodactyla are intimately 

 connected by numerous fossil forms. 



Order XI. — Cetacea. 



The porpoise has been described as a typical aquatic 

 mammal and it also serves as a type of the order Cetacea. 



Under the heading of the porpoise we have noticed the 

 adaptations to an aquatic life which constitute the main 

 peculiarities of the Cetacea. These consist of the follow- 

 ing :— 



1. Fish-like shape, with dorso-ventral coloration. 



2. Loss of hair and external ears and formation of 

 ^* blubber." 



3. Fore-limbs formed into fins, hind-limbs lost and tail 

 forming a fin. 



4. Homodont dentition (fish diet). 



5. Modification of nostrils to form vertical blowhole and 

 prolongation of larynx. 



6. Retia mirabilia. 



7. Loss of salivary and lacrymal glands. 



In addition, we may note the well-convoluted cerebrum 

 of the brain and the abdominal testes. The stomach is 

 usually somewhat complex, though the whole order is essen- 

 tially carnivorous — an important distinction from the Sirenia. 

 The uterus is bicornuate and the placenta, like that of many 

 Ungulata^ is diffuse and non-deciduate. 



