386 MURIDiE— MICROTIN^ 



dentinal pulps gradually atrophy, and in so doing form two 

 tapering roots — an anterior and a posterior — to each tooth. 



During their progress towards hypsodonty and persistent 

 growth the inner and outer triangular cusps have been con- 

 verted into triangular prisms, the structure of which partially 

 explains the final steps in evolution of the cheek-teeth of the 

 higher Microtince. Each triangle is bounded before and behind 

 by enamel (Fig. 57, e); with a thick internal lining of hard and 

 dense dentine (d) and a central core of a softer, more highly 

 vascular tissue, the osteo-dentine (0). Owing to its superior 

 powers of resistance the enamel always stands out in relief 

 upon the triturating surface of the teeth. In lower forms, as 

 Evotomys (Fig. 63), the enamel is rather thick, and, as in primi- 

 tive mammals generally, of uniform thickness at practically all 

 points upon the margins of the teeth. In the higher forms, i.e. 

 those like Microtus in which the power of subsisting upon a 

 coarse and tough vegetable aliment reaches its highest expres- 

 sion, the cutting and slicing functions of the cheek-teeth are most 

 completely developed, and in order to put them in operation 

 the motion of the jaw becomes almost exclusively an antero- 

 posterior one. In consequence of this, certain portions of the 

 enamel become of diminished utility and tend to become thin ; 

 other portions, directly obstructing the antero-posterior motion 

 of the jaw, atrophy and disappear. In most living Microtinm, 

 therefore, the enamel has become differentiated into thin and 

 thick portions ; the thick is usually found on the concave 

 sides of the prisms, which are, in upper teeth, the posterior, 

 in lower, the anterior ; the thin enamel forms the convex sides ; 

 further, in the anterior or posterior "loops" of mx and w^ 

 and at the tips of the salient angles, the enamel is frequently 

 lacking. As in other primitive mammals, the valleys or infolds 

 were originally devoid of cement, and this condition is still met 

 with in some genera which in many other ways seem to stand 

 as high or higher than the majority of their allies, e.g., in 

 Dicrostonyx (Fig. 55) and Ellobius ; in other genera cement 

 is found partially or wholly filling the re-entrant folds or 

 cement-spaces (Figs. 57, 63, etc.). 



The reduction in the number of primitive elements character- 

 istic of the ancestral cheek-teeth has not been limited to 



