498 CHORD ATA, 



germs and the deciduous premolar together as the lacteal 

 or deciduous series and the replacing premolar, together 

 with the other functional teeth, as the permanent series. 

 The difference between the Metatheria and Eutheria in 

 their dentition would then resolve itself into one of degree 

 only, the former having reduced their lacteal dentition till 

 only vestiges of all but the last remain. This reduction 

 might be correlated with the great development of the 

 lacteal nutrition involving a sucking mouth and loss of 

 function for teeth till a later period in life. 



Other structural features of the Metatheria are as 

 follows : — 



There is a prolonged period of mammary gestation, 

 during the early part of which the young are fed by the con- 

 traction of muscles over the mammary glands, the milk being 

 injected down the throat of the young. In a large number 

 of the Metatheria a fold of the abdominal integument 

 envelops the young, forming a pouch or marsupium. The 

 teats are long and are always abdominal in position. 



The brain is small in proportional size and has a large 

 anterior commissure but a small corpus callosum, as in 

 Prototheria. The skull of a metatherian may be known by 

 the following peculiarities, of w^hich the majority are usually 

 present : — 



1. The angle of the mandible is inflected. (See Fig. 349.) 



2. The lacrymal foramen is outside the orbit. 



3. The malar extends backwards to the glenoid cavity. 



4. The bony palate is incomplete. 



The inflected mandibular angle is probably a trace of the modifica- 

 tion by which the quadrate bone has become the tympanic, the malar 

 probably in early types extending back behind the squamosal to the 

 quadrate (see ear-ossicles). The lacrymal foramen was probably 

 primitively outside the orbit, and the complete bony palate is a mam- 

 malian character, its incompleteness hence indicating an early type. 



These skeletal features may be illustrated by taking the 

 kangaroo as a type of the Metatheria 



The Kangaroo {Macropus). 



The kangaroo belongs to the order Diprotodontia or 

 herbivorous section of the Metatheria. 



