CHAPTEE VII 



THE SUBCLASS EUTHERIA AND THE ORDER EDENTATA 



The whole of the remaining groups of mammals are included in a 

 single subclass, known by the names Eutheria, Monodelphia, or 

 Placentalia. 1 The one distinctive feature they have in common 

 (from which the last-mentioned name is derived) is the presence of 

 an allantoic placenta by means of which the foetus is nourished within 

 the uterus of the mother. Throughout the entire subclass, as a general 

 rule, the urino-genital organs open quite independently of the rectum ; 

 the corpus callosum of the brain is well developed ; the mandible does 

 not show a marked inflection of its angle ; and distinct epipubic 

 bones are not attached to the anterior margin of the pubic symphysis. 

 In those cases where there is a heterodont and diphyoclont dentition 

 the dental formula can be reduced to some modification of the one 

 given on p. 25, there being only one known genus Avhere four 

 true molars occur, and even that not invariably. As in the 

 Metatheria, the coracoid is reduced to a mere appendage of the 

 scapula, and the acetabular cavity of the pelvis is imperforate. 

 While the survivors of the other subclasses have probably been 

 for a long time in a stationary condition, these have, as there is 

 already good evidence to show throughout all the Tertiary 

 geological age, and by inference for some time before, been multi- 

 plying in numbers and variations of form, and attaining higher 

 stages of development and specialisation in various directions. 

 They consequently exhibit far greater diversity of external or 

 adaptive modification than is met with in either of the other sub- 

 classes, — some being fitted to live as exclusively in the water as 

 fishes, and others to emulate the aerial flight of birds. 



To facilitate the study of the different component members 

 of this large group, it is usual to separate them into certain 



1 The characters of the chief groups of the Eutheria here given are, in some 

 measure, a fuller recapitulation of those already detailed in Chapter III., pp. 

 83-88. 



