Prototlieria sufficiently to show that they are distinctly below 

 any other groups of the Mammalia. 



The Marsupialia may also be regarded as the existing 

 representatives of an ancient group which diverged from the 

 ancestral stem far back, but above the origin of the 

 Prototheria. This group is Huxley's Metatheria ; while all 

 the remaining Mammalia, the descendants of the ancestral 

 series, after the separation of the Prototheria and Metatheria, 

 are the Eutheria. 



The existing Marsupials show in their anatomical charac- 

 ters a condition intermediate between that of the Prototheria 

 and that found in the Eutheria. The mammary glands in 

 particular are much more highly developed than they are in 

 the Monotremata; but in the structure of the brain and some 

 parts of the skeleton they are still distinctly below the other 

 groups of the Mammalia. The Marsupialia, although 

 retaining the essential characters of the Metatheria, have pro- 

 bably undergone considerable evolution, which has affected 

 the less important points of structure, and given rise to 

 several distinct groups presenting differences mainly in 

 external features, and evidently related to their very diverse 

 habits. 



In the evolution of the early Eutheria, the third group of 

 ancestral Mammalia,"^' one of the most important changes 

 was the acquisition of the allantoic placenta for the nourish- 

 ment of the embryo. The placenta is present in all of the 

 Eutheria, which are therefore called '* Placental Mammals," 

 and is variously modified in the different groups.! 



The Edentata were probably the first important divergent 

 series from the ancestral Eutheria. The existing members 



* For an account of the Mammalia see Flower's article *' Mammalia," in 

 Encijcl. Brit., 9th edition, v. xv, p. 347. 



fFor a comparative history of the Placenta, see Balfour, Com]). 

 KmhnjoL, v. ii, p. 198. 



