102 MICROTIN^ 



2. The Evolution oj the Cheek-Teeth. 



The problems relating to the evolution of the cheek-teeth of 

 the Microtinse are fascinating but difficult. In all members of 

 the subfamily the molars are hypsodont and prismatic, and 

 the worn surfaces of these teeth are always fiat and even. In the 

 highest genera the molars are persistently growing teeth implanted 

 in the jaws by their crowns and not by fangs ; the re-entrant folds 

 of the crowns are partly filled by cement, which gives attachment 

 to the alveolar periosteum ; the crowns show a double system 

 of curvatures, one more or less transverse, the other longitudinal. 

 By means of the alveolar periosteum and the curvatures of their 

 crowns in combination, the teeth are kept tightly " keyed-up " 

 together at the grinding surface, and the delicate dentinal 

 pulps and enamel organs at the bases of the crowns are protected 

 from the injurious effects of pressure and other stresses and strains 

 when the teeth are in use. In the highest Microtinse the 

 structure of the crown itself shows an important peculiarity. The 

 enamel, which in primitive Muridse is rather thick, continuous, 

 and at almost all points of the periphery of uniform thickness, is 

 in these highly developed teeth thinner generally and differentiated 

 in a peculiar way; on the concave sides of the prisms or salient 

 angles, i.e., on the posterior sides of the prisms in upper, the 

 anterior sides in lower molars, the enamel is relatively thick, 

 whereas on the convex sides it is very thin, and at certain points 

 it may be lacking altogether. When the molars are at work 

 the lower jaw moves forwards so that the opposed plates of thick 

 enamel, curved in opposite directions, backwards above, forwards 

 below, shear past each other like scissor blades ; thus the tough 

 vegetable matter upon which the higher voles feed is sliced and 

 cut to pieces by the cheek-teeth. 



It is generally recognized that the Microtinae have descended 

 from the same ancestors as the rest of the Muridee; therefore 

 it is admitted that the highly specialized cheek-teeth of the 

 Microtinae have been evolved gradually from the primitive 

 brachyodont and tuberculate molars which characterize all 

 lowly Muridae. The hypsodont cheek-teeth of the Microtinae 

 are adapted, as already stated, for the shearing and slicing of 

 coarse, tough vegetable substances; but the brachyodont 

 tubercular molars of lowly Muridae, with their crowns of limited 

 growth, uneven wearing surfaces, and undifferentiated enamel 

 are fitted merely for the purpose of bruising and crushing the 

 relatively soft and succulent materials which form a mixed diet. 

 Microtinae owe directly or indirectly their special characters and, 

 having regard to the severe competition which they have had 

 to face with the Murinae in the Old World and with the Cricetinse 

 in the New World, probably their continued existence, to the 

 circumstance that they have acquired the power of subsisting 

 upon food which is despised for the most part by their rivals. 



