I.] DIDELPHIA. 5 



they agree with the former, while in the structure of the 

 earbones and the shoulder-girdle, and the presence of 

 true teeth, and of teats on the mammary glands, they 

 resemble the latter; the reproductive organs belonging to 

 neither one nor the other type, but presenting a special 

 character representing an intermediate grade of development. 

 The ureters open into the base of the bladder. The 

 oviducts are differentiated into uterine and Fallopian por- 

 tions, 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 

 transferred to the nipple of the mother, to which they remain 

 firmly attached for a considerable time, nourished by the milk 

 injected into the mouth by compression of the muscle cover- 

 ing the mammary gland. They are, therefore, the most 

 typically mammalian of the whole class. The nipples are 

 nearly always concealed in a fold of the abdominal integu- 

 ment or "pouch" (marsupium), which serves to support 

 and protect the young in their early helpless condition. 

 The existing species of this group are entirely confined to 

 the Australian region and the American continent, though 

 in former times they had a more extended geographical 

 range. The earliest Mammals hitherto discovered appear 

 (as far as the scanty evidence at present obtainable permits 

 any such conclusion to be hazarded) to have belonged to 

 this type, although it is reasonable to conclude that 

 Prototheria (unless upon the improbable supposition that 

 the existing forms have resulted from a process of degrada- 

 tion), and perhaps Eutheria, were their contemporaries far 

 back in the Mesozoic age. 



