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THE AMERICAN NATURALIST [Vol. XLII 



circle. This is beautifully illustrated by Fig. 1, in which 

 the right incisor has been removed from the jaw. 



Both teeth are turned somewhat toward the right, so 

 that the right one projects from the mouth ; but the left 

 incisor strikes the roof of the mouth to the right of the 

 median suture, piercing the palatine plate of the maxilla 



(Fig. 2) and extending through it to a distance of about 

 5 mm. The left tooth is not as long as the right one, its 

 growth having been retarded, perhaps by the hardness 

 of the bone it penetrated. 



The manner in which the teeth of Marmota rnonax 

 grow is familiar to all who know the Rodentia. The 

 rodents all have a diphyodont dentition, that is, there are 

 two sets of teeth, a temporary or deciduous set, and a 

 permanent set. But the permanent teeth never cease to 

 grow. There is a persistent pulp at the base of each 

 tooth, which grows throughout the life of the individual. 



Ordinarily the corresponding teeth of the upper and 

 lower jaw oppose each other perfectly, and the growth 

 from the pulp only compensates for the amount worn off 

 by biting. The incisors have a heavier coat of enamel on 



