498 CHORDATA. 
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 nlarsupium. 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 which 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 Ale¢atheria. 
THE Kancaroo (Macropus). 
The kangaroo belongs to the order Dzprotodontia or 
herbivorous section of the AMefatheria. 
