50 Field Museum of Natural History — Zoology, Vol. XL 



always abdominal. In the male the usual positions of the external 

 genital organs are transposed; a caecum is present, but small. 



Marsupials possess a number of characters which have puzzled 

 naturalists as to their origin, as in some ways they resemble Prototherian 

 mammals and in others Eutherian, but the balance of evidence seems 

 to indicate a much nearer relationship to the latter, and it is probable 

 that they are derived from some primitive form of Eutherian mammal, 

 having, in Beddard's opinion, "separated from the Eutherian stock after 

 it had acquired a definite diphyodonty and the allantoic placenta."* 



Mammalia, Kjo. 



Skull of Opossum (Didelphis virginiana). 

 (About .^ nat. size.) 

 p. 119. 



