628 LAND MAMMALS IN THE WESTERN HEMISPHERE 



served. There is a difference of opinion among naturalists 

 as to the proper interpretation of the marsupial dentition. 

 According to one view, all except the replaced premolar belong 

 to the milk-series and the permanent series has been lost ; 

 the alternative and more probable belief is that the milk- 

 dentition has been almost or completely suppressed. Which- 

 ever one of these interpretations be the right one, there is 

 strong reason to maintain that the very limited amount of 

 change is not a primitive condition, but a secondary one, for a 

 series of rudimentary teeth is formed before those which are 

 to become functional. The only reasonable explanation of 

 such a condition is that it has been derived from one in which 

 the normal succession and replacement of the teeth took place. 

 Something of the same sort has been observed in the sim- 

 plicidentate rodents. The marsupial dentition differs from 

 the placental one in the usual number of four molars, in- 

 stead of three, and frequently also in exceeding the normal 

 total number of 44. The incisors are almost always of a 

 different number in the upper and the lower jaw and are fre- 

 quently more numerous than in the placentals. 



The skeleton has several diagnostic characters, which are 

 present throughout the order, though one or other of these 

 features may be absent in particular instances. The skull has a 

 very small brain-capacity and elongate face and jaws. In the 

 placental mammals, the sutures between adjoining bones of the 

 skull tend to close by coossification, and the separate bones 

 are clearly distinguishable only in young animals ; but in the 

 marsupials the sutures remain open for a much longer period. 

 The lachrymal is expanded on the face and the foramen is 

 outside of the orbit. The tympanic is a mere ring and per- 

 manently separate from the other bones of the cranium, while 

 a false bulla is formed by the inflation of part of the alisphenoid. 

 In almost all marsupials there are large openings or vacuities 

 in the bony palate. One of the most characteristic and con- 

 stant features of the marsupial skull is in the conformation of 



