MAMMALIA, 495 
mesozoic period from the Trias onwards and have doubtful 
claims to be regarded as Prototheria. These claims rest 
chiefly upon the resemblance of their molar teeth to those 
of Ornithorhynchus. They have, however, large incisors, 
one pair being much larger than the rest. The heterodont 
condition is therefore already present. Still more doubtful 
are the supposed vestiges of a coracoid and episternum. It 
is obvious that nothing is known of the soft parts, but if 
their skeleton were shown to agree closely with that of the 
Monotremata there would be reasons for assuming that 
they probably also possessed the three first features of the 
Prototheria. 
Sub-Class II.—Metatheria. 
The Metatheria have two living orders, the Diprotodontia 
and the Polyprotodontia. They may be said to present 
at least five important sub-class characters :— 
1. They are viviparous but have a very short period of 
uterine gestation, during which a yolk-sac placenta is 
present and an allantoic placenta only exceptionally. 
2. The oviducts are divided into three parts—(x) Fallo- 
pian tube, (2) uterus, (3) vagina, and there is no fusion 
between the oviducts except at the lower part of the 
vagina. 
3. Urogenital sinus and rectum open separately to the 
exterior, though surrounded by the same sphincter muscle. 
4. Amongst numerous skeletal peculiarities we may note 
the presence of epipubic bones and of only one deciduous 
tooth on each side of each jaw. 
5. The temperature is more constant thanZin Proto- 
theria, but is lower than in Eutheria. 
The condition of the placenta has been described. The 
allantois is obviously in a degenerate condition in the 
majority of Metatheria (of. Hypsoprymnus), but in forms like 
Phascolarctos it is normal and possibly performs its primary 
function of respiration. 
Recently, however, the discovery of an allantoic placenta 
in Perameles has shown us that at least one metatherian has 
advanced to the foetal condition of the Eutheria. The 
structure of this placenta would seem to have certain 
