128 Troxell — Rodents of Genus Ischyromys. 



of ridges clearly the result of lateral wear ; furthermore, 

 the upper teeth are often worn to the roots on the inner 

 side while the outer borders still stand high. This gives 

 evidence that the movement of the jaws was transverse 

 rather than longitudinal as in the case of many rodents 

 where the grinding surface as a unit is worn to 

 one smooth plane (cf. the water hog, Hydrochcerus 

 capybara). 



Fig. 6. — Upper milk teeth of Ischyromys typus Leidy, Cat. No. 12508, 

 Y. P. M. Dp 3 is very much like the premolar which succeeds it. Dp 4 is 

 short-crowned, but elongated fore and aft. Note that the first molar faces 

 outward. X 2 - 



The third superior molar is set in the maxillary 

 facing downward and outward at an angle of more than 

 40°, but the deviation from the vertical is progressively 

 less toward the anterior teeth ; the lower teeth correspond- 

 ingly face upward and inward. In the action of chewing, 

 therefore, the first contact is formed by the vertically 

 placed premolars, then, as the lower teeth move inward, 

 the molars from front to rear successively come into use. 



Taking each tooth separately, its action may be ana- 

 lyzed into a chopping process with the first contact of the 

 sharp grooves and crests; this becomes secondarily a 

 tearing or grinding where two teeth are fully opposed ; at 

 that phase the inner tubercles of the upper tooth and the 

 outer cusps of the lower tooth resist further lateral 

 movement, thus causing additional grinding of the food 

 particles. 



The milk teeth, like the premolars in their early stages, 

 stand vertical both above and below, but the wear of these 

 teeth indicates that the immature individual has already 

 learned the transverse movement of the jaws. There are 

 two milk teeth above and one below in the young animal. 



Relationship and adaptations. — A most interesting 

 comparison may be made between Ischyromys, the fossil, 

 and Cynomys, the prairie dog, and although certain fea- 

 tures stand out sharply separating the two genera, yet 

 they show a fundamental similarity which tells of close 

 relationship. The skull of Cynomys (Cat. No. 01228, 



