590 



PROF. F. WOOD JONES ON THE 



Upper Molars. 



The upper molar series when examined from its occlusal sur- 

 face presents a remarkably simple pattern in the young animal 

 when practically no signs of attrition are present, and the same 

 simplicity is preserved in the adult when attrition is well advanced 

 (see text-fig. 3, A). The anterior molar consists of three very 

 distinct masses arranged in the antero-posterior axis of the tooth. 



Text-figure 3. 



Left upper molar series of Hydromys chrysogaster. A, showing tlie occlusal 

 surface of the two molars ; and B, the alveolar cavities. The outlines of the 

 anterior molar are superimposed on its alveolar cavities in B. The cavity- 

 marked X is not present in some specimens. 



Each of these divisions shows a central depressed area when 

 viewed from the occlusal surface. 



The posterior molar consists of a small anterior lingual portion 

 and a larger posterior mass. The total area of the posterior 

 molar is less than half that of the anterior tooth. (See text- 

 fig. 3.) In advanced age the wear of the tooth involves the raised 

 fim of the individual crown masses, and beyond a flattening of 



