MAMMALIA. 469 
vertebra and hence enveloping the artery in a complete 
an ring. (Camels form a remarkable exception to this 
rule. 
The thoracic vertebree 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 detween the 
vertebrae (¢f chevron-bones), hence the half-facet. The 
transverse process also has a facet for the tuberculum. In 
many thoracic vertebree the neural spines are very long. 
The thoracic vary in number throughout the orders. 
The lumbar vertebree 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 vertebree. 
The sacrum is formed of two primary sacral vertebre 
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 Lutheria one 
or more caudal vertebree more or less welded into the 
sacrum. 
The caudal vertebree 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 Svvenza and Cefacea, the tail is hyper- 
trophied and the vertebrz, as also in some terrestrial forms, 
é.g., Kangaroo, bear chevron-bones or'ventral arches articulat- 
ing between the centra. Ina 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 America 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 Axthropoidea the tail is vestigial, reduced to 
half-a-dozen fused vertebra called the coccyx, which no 
