MAMMALIA 507 
and of multiplication of molars in others by secondary divi- 
sion, makes it possible to derive the more aberrant types. 
Of these we may instance the Adentata and Cefacea as 
differing widely from the type. The typical dentition indi- 
cates two very important differences from the Metatheria. 
Firstly, the incisors are never more than three on each side 
and, secondly, the molars are not more than three. We have 
seen that four incisors and four molars are the rule in the 
Polyprotodontia and that four molars are usual in the Dzpro- 
todontia. Lastly, we may call to mind the peculiar condition 
of the deciduous or milk-dentition in the Mefatheria. A 
complete milk series (diphyodont) is the rule in Lu¢heria. 
Turning to the rest of the skeleton we find that, as in 
Metatheria, the coracoid element of the shoulder-girdle is 
reduced to a mere vestige; the coracoid process of the 
scapula, and the episternum is absent as a separate bone. 
In the pelvic girdle there are no epipubic bones. 
The temperature of Zu¢heria is higher than that of either 
Metatheria or Prototheria and is also more constant ; that 
is to say, the temperature of the body varies only within 
narrow limits whatever the temperature of the surroundings. 
This is only another instance of the higher type having its 
internal economy adjusted in such a way as to be inde- 
pendent of the immediate surroundings. The individual 
variations within the sub-class are from about 35°C. to 
40°C. 
Modern £u¢heria have not only an important structural 
distinction in their brain from that of the other sub-classes, 
such as the great development of the corpus callosum and a 
corresponding reduction in the anterior commissure, but 
also an advance in the type of brain. It usually forms a 
greater proportion of the bulk of the body, the cerebrum 
gradually assuming more and more comparative importance 
as the higher orders are reached. Thus the cerebrum comes 
to completely overlie not only the optic lobes but the cere- 
bellum as well, and its surface becomes folded into numerous 
convolutions. Apparently the earlier fossil forms (ég., 
Eocene) had far smaller brains in proportion, and a 
gradual increase in size and complexity of the brain there- 
fore appears to be one of the most important lines along 
which mammals have progressed. The exact significance of 
