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 Edentata and Cetacea 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 Dipro- 

 todontia. Lastly, we may call to mind the peculiar condition 

 of the deciduous or milk-dentition in the Metatheria. A 

 complete milk sejies (diphyodont) is the rule in Eutheria. 



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 oi Eutheria 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 Eutheria 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 (e.g., 

 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 



