630 LAND MAMMALS IN THE WESTERN HEMISPHERE 



The new-bom young are transferred to the nipples of the mother 

 and are attached to these and fed by the pumping of milk into 

 their mouths by muscular action of the mother. A special, 

 though temporary, arrangement of the gullet and windpipe 

 is provided, so that the helpless young animal shall not be 

 suffocated by the entrance of milk into the lungs. 



Suborder Polyprotodonta 



This suborder, as is indicated by its name, is characterized 

 by its numerous incisors, which are f , or f , and none of them 

 is especially enlarged ; by the large canines in both jaws, simple 

 premolars and tritubercular upper molars. The members of this 

 group are carnivorous or insectivorous in habit, and all the 

 existing ones are of small or moderate size, though some very 

 large extinct forms are known. Except in one Australian 

 family, the feet are not "syndactyl," a term which means the 

 enclosure of two or more digits in one fold of skin. The only 

 existing American representatives of the suborder are the 

 opossums, the great majority of which are Neotropical in dis- 

 tribution. 



1. Didelphiidce. Opossums 



In this family the dental formula is : i|, Cy, p|, mf , X 2 = 50. 

 The incisors are small and closely crowded together, the canines 

 large and tusk-like, the premolars simple and of compressed- 

 conical form ; in existing species, the upper molars are triangular, 

 each of the three main cusps is V-shaped and there are addi- 

 tional minute cusps along the outer border ; the lower molars 

 have a high anterior triangle of three pointed cusps and a low 

 heel with several distinct cusps. The humerus has an epicon- 

 dylar foramen and the feet are five-toed ; in the manus all the 

 digits are armed with claws and the thumb is but partially 

 opposable, while in the pes the hallux is without a claw and 

 completely opposable, making the foot much like that of a 

 monkey. 



