386 MURIDAE—MICROTINZE& 
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 A/zcrot7ne. Each triangle is bounded before and behind 
by enamel (Fig. 57, ¢); with a thick internal lining of hard and 
dense dentine (¢) anda 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, ze. 
those like J/zcrotus 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 Mzcrotine, 
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 mz, and m2’, 
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.¢., in 
Dicrostonyx (Fig. 55) and £2/odzus ; 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 
