50 Fierp Museum or Natura History — Zootoey, Vor. XI. 
always abdominal. In the male the usual positions of the external 
genital organs are transposed; a cacum 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 afer 
it had acquired a definite diphyodonty and the allantoic placenta. 
Skull of Opossum (Didelphis virginiana). 
(About } nat. size.) 
* Mammalia, 1902, p. 119. 
