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 — (i) 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 than in 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 {cf. 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 



