I2i 



CHAPTER LX. 



CLASS MAMMALIA— continued. 



Orders Edentata, Cetacea, and Sirenia. 



Subclass II. Eutheria. — The whole of the remaining orders of 

 Mammalia are grouped together in a single subclass, 1 which is 

 characterised by the foetus being nourished in utero by means of the 

 maternal blood passing through an allantoic placenta. This sub- 

 class is sometimes termed the Placentalia, but more generally the 

 Eutheria. Throughout this subclass the urinogenital organs are 

 provided with an external aperture quite distinct from that of the 

 alimentary tube ; the corpus callosum of the brain is well developed , 

 there is never any marked inflection of the angle of the mandible ; 

 and distinct epipubic bones are absent in the pelvis. With the ex- 

 ception of the three orders forming the subject of the present chap- 

 ter, the dental formula can always be reduced to some modification 

 of that given on page 1249. At the present day the various orders 

 have become so well differentiated as to render their definition com- 

 paratively easy ; but fossil forms indicate such a close connection 

 between the majority of them, that such definition becomes fre- 

 quently a matter of extreme difficulty, if not an absolute impossi- 

 bility ; and it is to be remembered that it is entirely due to our non- 

 acquaintance with forms which must have once existed that renders 

 even these imperfect definitions practicable. 



There is at present no conclusive evidence of the existence of any 

 member of this subclass previous to the Eocene. 



Order III. Edentata. — The Edentata are widely different from 

 all other existing Mammals, although there are indications of affinity 

 to certain extinct forms mentioned in the sequel under the heading 

 of the Tillodontia. Almost the only common character presented 

 by the various existing members of this order is that the teeth, when 



1 It has, indeed, been proposed to form a separate subclass — Paratheria — for 

 the reception of the Edentates. 



