EXTERNAL CHARACTERS OF THE KOALA, 



605 



From the description of these three pouches it is evident that 

 the pouch of Phascolomys is structurally intermediate in many- 

 respects between those of Trichosurus and Phascolarctos. The 

 differences between the pouches of the last two genera, consider- 

 ing that both are ai-boreal forms, is remarkable, especially in view 

 of the theory that has been advanced that the anterior upward 

 aspect of the orifice is an adaptation to arboreal life or to the 



Text figure 26. 









A. Pouch of Phascolarctos cinereiis. 

 V>. „ Fhascolomys ursinus. 

 C. ,, Trichosurus vulpecula. 



The extension of the pouch beneath the integument with the mammoB {m) 

 at its posterior end shown diagrammaticallj- with dotted lines. 



more or less upright attitude assumed by such teirestrial bipedal 

 forms as Macrojms, and conversely that the posterior aspect of 

 the orifice is an adaptation to the terrestrial life and quadru- 

 pedal gait of such genera as Thylacinus and Sarcophilus. This 

 theory" entirely breaks down in the case of Phascolarctos, per- 

 haps the most specialized scansorial genus in the entire order. 



