32 MICROTIN^ 



to glide forwards when the molars were pressed tightly together ; 

 but as the forward stroke became firmly established the crown 

 tubercles gradually ceased to be functional parts of the crowns. 

 The enamel, primitively rather thick, equally developed, and 

 continuous in all parts of the periphery of the crown, became 

 differentiated into thick and thin portions, remaining thick where 

 required, becoming thin or disappearing altogether in situations 

 where it was no longer useful or impeded the stroke of the teeth. 

 In this way the enamel has been re-arranged in upper and lower 

 molars to form a series of appropriately curved cutting blades 

 which shear with each other effectively as the lower jaw is pulled 

 forwards and upwards by the muscles. The apical enamel has 

 atrophied and disappeared to a very large extent, so that the 

 molars of many voles now present as soon as they cut the gum 

 a flat surface upon which hard enamel and soft dentine are already 

 exposed in effective alternation. Step by step, as the food has 

 increased in harshness, the rate at which the substance of the teeth 

 is wasted by attrition has become more rapid; in compensation 

 the crowns of the teeth have become progressively taller, their 

 dentinal pulps and enamel organs more vigorous and more con- 

 tinuously active; and the loss of that vigour and activity has 

 been postponed to a later and later moment in the life of the 

 individual from generation to generation, until at last in the highest 

 Microtinse the molars like the incisors have acquired the power of 

 persistent growth. 



Other direct effects of the change of diet are modifications in 

 the alimentary canal. The food of the Microtinse is richer in 

 cellulose than that of less specialized Muridte and the process of 

 digestion is chemically somewhat different. This has led, as in 

 other rodents which subsist upon similar food-stuffs, to marked 

 enlargement of the caecum and to enlargement and complication 

 of the large intestine.^ 



The increased height of the molar crowns and their robust 

 proportions have necessarily led to the enlargement of the alveolar 

 capsules in which the teeth are developed and supported. In the 

 upper jaw the capsules rise up as conspicuous swellings in the 

 floors of the infraorbital canal and nasal chamber (m^) and in the 

 floors of the orbit and sphenorbital fissure (m^ and m^) ; in the 

 lower jaw they fill the body of each horizontal ramus and with 

 the contiguous shaft of the lower incisor impart to the mandible 

 a characteristic robustness of form. In those less specialized 

 Microtinse in which the molar teeth still develop roots in old age, 

 senility is marked by the gradual subsidence and collapse of the 

 alveolar capsules ; in these forms, therefore, the alveolar portions of 

 the jaws revert in old age to the condition seen throughout life in 

 primitive Muridse. But in all Microtinae the great size of the 

 alveolar capsules, whether a permanent or a more or less temporary 

 feature, has produced important modifications of the surrounding 

 ^ TuLLBERG, Ueber d. System der Nagetiere, p. 443. 



