CHAPTER Vl 



INTRODUCTION TO THE SUB-CLASS EUTHERIA 



Sub-Class II.—EUTHEIIIA 



Definition. — Mammalia with teats. Mammary giauds of seba- 

 ceous type. Heart with entirely membranous and complete right 

 auriculo-ventricular valve. Brain generally with a corpus cal- 

 losum. Coracoid much reduced and not reaching sternum. No 

 interclavicle. Vertebrae with epiphyses. Kibs double-headed. 

 Viviparous, with a small ovum. 



In this group are included not only the Eutheria in the sense 

 of Huxley, but also his Metatheria. Though the Metatheria, or 

 Marsupials as we shall term them, undoubtedly form a most 

 distinct order of mammals, perhaps even a trifle more distinct 

 than most others, their differences from the remaining tribes are 

 not by any means so great as those which separate OrnitJio- 

 rhynchus and Ecltidna from all other mammals. In his well- 

 known memoir upon the arrangement of the Mammalia,^ Pro- 

 fessor Huxley enumerated eleven characters as distinguishing the 

 Metatheria either from the Prototheria or from the Eutheria. 

 Of these only three were characters in which they approach the 

 lower mammals According to his showing, therefore, the 

 preponderance of marsupial features are Eutheriau. The three 

 characters of Prototherian type are (1) the presence of epipubes ; 

 (2) the small corpus callosum ; (3) the absence of an allantoic 

 placenta. 



The last of these can be dismissed, in consequence of the 

 recent discovery of an allantoic placenta in Perameles. The first 

 character is apparently a valid distinction between the Marsupials 

 1 Proc. Zool. Soc. 1880, p. 649. 



