CHAP Val 
INTRODUCTION TO THE SUB-CLASS EUTHERIA 
SuB-CLASS I1—EUTHERIA 
Definition. Mammalia with teats. Mammary glands 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. Ribs 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 Ornitho- 
rhynchus and Eehidna 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 Eutherian. 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, im consequence of the 
recent discovery of an allantoic placenta in Perameles. The first 
character is apparently a valid distinction between the Marsupials 
l Proc. Zool. Soc. 1880, p. 649. 
