CHAPTEE VI 



THE SUBCLASS METATHERIA OR DIDELPHIA 



General Characters. — The Metatheria or Didelphia are represented at 

 present by numerous species, presenting great diversities of general 

 appearance, structure, and habits, although all united by many 

 essential anatomical and physiological characters, which, taken 

 altogether, give them an intermediate position between the Proto- 

 theria and the Eutheria. 



Although the striking differences in external form, in many 

 anatomical characters, and in mode of life of various animals of this 

 section might lead to their division into groups equivalent to the 

 orders of the Eutheria, it is more convenient on the whole to adhere 

 to the usual custom of treating them all as forming one order called 

 Marsupialia, 1 the limits of which are therefore equivalent to that 

 of the subclass. The more essentially distinctive characters are as 

 follows. 



In the structure of the brain and the presence of epipubic bones 

 they agree with the Prototheria, while in the structure of the ear- 

 bones and the shoulder- girdle and the presence of teats on the 

 mammary glands they resemble the Eutheria, the reproductive 

 organs belonging to neither one nor the other type, but having a 

 special character representing an intermediate grade of develop- 

 ment. The ureters open into the base of the bladder. The 

 oviducts are differentiated into uterine and Fallopian portions, and 

 open into a long and distinct vagina, quite separate from the cystic 

 urethra. The penis is large, but its crura are not directly attached 

 to the ischia. The spongy body has a large bifurcated bulb. The 

 young are born in an exceedingly rudimentary condition, and are 

 never nourished by means of an allantoic placenta, but are trans- 

 ferred to the nipple of the mother, to which they remain firmly 



1 For the detailed characters of all the genera and species of Marsupials the 

 reader should consult the British Museum Catalogue of Marsupialia and Afono- 

 tremata, by Old field Thomas, 1888. 



