MAMMALIA. 469 



vertebra and hence enveloping the artery in a complete 

 bony ring. (Camels form a remarkable exception to this 

 rule.) 



The thoracic vertebrae bear the functional ribs. They 

 may be known in mammals by the articular half-facet on 

 the centrum for the capitulum of the rib. As a rule in 

 mammals the capitula of the ribs articulate between the 

 vertebrae {cf. chevron-bones), hence the half- facet. The 

 transverse process also has a facet for the tuberculum. In 

 many thoracic vertebrae the neural spines are very long. 

 The thoracic vary in number throughout the orders. 



The lumbar vertebrae approximate at the anterior end 

 to the thoracic in character, but they have no free ribs. 

 The ribs are fused on to the transverse processes, thus 

 producing large flat lateral wings which are usually known 

 as " transverse processes." The neural spines are never 

 long in the lumbar vertebrae. 



The sacrum is formed of two primary sacral vertebrae 

 which are firmly welded together and to the ilium. They 

 also contain rib-elements in the short transverse processes 

 (still seen in crocodiles). There are usually in Eutheria one 

 or more caudal vertebrae more or less, welded into the 

 sacrum. 



The caudal vertebras vary enormously amongst mammals 

 in size and number, just as the ^'tail" also varies. They 

 are usually more or less simple rod-shaped bodies. In the 

 aquatic forms, such as Sirenia and Cetacea^ the tail is hyper- 

 trophied and the vertebrae, as also in some terrestrial forms, 

 e.g.^ Kangaroo, bear chevron-bones or ventral arches articulat- 

 ing between the centra. In a good number of mammals the 

 tail forms a valuable accessory limb, more especially in the 

 arboreal types. The muscles of the prehensile tail are 

 strengthened and the end of the tail is wound round a 

 bough sufficiently firm to bear the weight of the animal, 

 thus freeing the limbs for other purposes. The forests of 

 South x\merica present us with a remarkable abundance of 

 forms with prehensile tails, some examples being the spider- 

 monkeys, tree-porcupines, tree anteaters, opossum-rats and 

 opossums. 



In some Anthropoidea the tail is vestigial, reduced to 

 half-a-dozen fused vertebrae called the coccyx, which no 



