DENTAL CHARACTERS OF AUSTRALIAN RATS. 



591 



the occlusal surfaces no marked change of crown-pattern takes 

 place with increasing age. 



Upon removing the two upper molars in Hydromys a very- 

 curious arrangement of the alveolar cavities is seen. There are 

 either 8 or 9 separate sockets for the reception of roots, small or 

 large, of the anterior tooth, and two cavities for the roots of the 

 posterior tooth. (See text-fig. 3, B.) 



Of the 8 or 9 root-cavities for the anterior tooth 2 belong to 

 the anterior lamina of the crown, 5 or 6 belong to the middle 

 lamina, and 1 to the posterior lamina. 



Text-figure 4. 



Z ingua/ 



Anterior left upper molar of 'H.ydromys chrysogaster to show the relation of the 

 multiple roots to the crown divisions. In this example only two lingual roots 

 are present. A=anterioi-, P=posterior extremity of molar. 



The anterior root is large and shows at its extremity a tendency 

 to be itself subdivided into 3. Four very minute labial cavities 

 belong to four small labial roots of the medial lamina and one or 

 two far larger lingual root-cavities belong to the same subdivision. 

 The small posterior portion of the anterior molar has one large 

 alveolus for a single large root which shows a marked tendency to 

 be bifid. (See text-figs. 3 & 4.) 



The two roots of the posterior tooth are arranged in the long- 

 axis of the jaw and thus both tend to be elongated from side to. 

 side and to be bifid. 



