CHAPTER 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 
Marsvupia.i4,! 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 Mono- 
tremata, by Oldfield Thomas, 1888. 
