442 THE VERTEBRATE SKELETON. 



The Exoskeletal structures of mammals may be summarised 

 in the following table : 



I. Epidermal exoskeletal structures. 



1. Hairs (a) ordinary hair, 



(6) vibrissae and bristles, 



(c) spines of hedgehog, porcupine, Echidna, 

 Centetes, Acanthomys. 



. ( of Manidae, 



2. Scales \ , 



[ on tails of rats, beavers, &c. 



3. Horns of Rhinoceros. 



4. Horns of Bovine Ruminants. 



5. Nails, claws, hoofs. 



6. Spurs of male Ornithorhynchus and Echidna. 



7. Horny beak and teeth of Ornithorhynchus. 



8. Horny pads on jaws of Sirenians and Ruminants. 



9. Baleen of whales. 

 10. Enamel of teeth. 



II. Dermal exoskeletal structures. 



1. Dentine and cement of teeth. 



2. Bony scutes of Armadillos. 



ENDOSKELETOK 



VERTEBRAL COLUMN. 



CERVICAL VERTEBRAE. 



The cervical vertebrae of all mammals have certain cha- 

 racters in common. However long the neck may be, the 

 number of cervical vertebrae, with very few exceptions, is 

 seven. Movable ribs are generally absent, and if present 

 are small and do not reach the sternum. The transverse 

 processes are generally wide but not long, and are perforated 

 near the base by the vertebrarterial canals, through which 

 the vertebral arteries pass ; they generally bear downwardly- 

 directed inferior lamellae which are sometimes as in the seventh 

 human cervical seen to ossify from centres distinct from those 



