402 MURIDA—EVOTOMYS 
of the cutting surface of the cheek-teeth nor forming a pro- 
tuberance on the outer surface of the mandible. 
The cheek-teeth are small, narrow, weak, and, in the young, 
rootless. Their growth from a rootless to a rooted condition 
causes great changes in the enamel pattern, which gradually loses 
its definiteness, and finally disappears. In the young teeth the 
dentine-spaces extend to the base; at which a ring forms later, 
then contracts and divides into two, each half becoming a broad 
root as the tooth is pushed upwards. When immature the 
enamel is thin, the salient angles sharp, the infolds wide and 
shallow, and consequently the dentinal spaces confluent. 
When adult the enamel is thick, the tips of the salient angles 
rounded, and the infolds deeper. The elements of the teeth are 

my Ms Ms 
(2) (3) (4) 
Fic. 61.—(1) ANTERIOR LEFT LOWER CHEEK-TOOTH OF Arvicola amphibius, tilted so as 
to show open pulp cavities. (2) Lert LOWER CHEEK-TEETH OF Lvo/omys glareolus, 
both seen from outer side. (3) &. giareolus, simple form of m*. (4) Z. skomerensis, 
complex m°. (Drawn by M. A. C, Hinton.) 

crowded longitudinally, so that the alternation of the inner and 
outer triangles tends to be inconspicuous ; further, the outer 
infolds of the lower teeth are usually shallow, so that each pair 
of prisms tends to form a transverse loop rather than an inner 
and an outer closed triangle. These features are characteristic 
of all the normal members of the genus. ’ and mm are of 
normal form, having the anterior loop followed by four and 
three substantially closed triangles respectively; the outer 
triangles are in each tooth slightly larger than the inner. In 
m® the anterior loop precedes two outer and one inner prisms, 
followed by a posterior loop of variable shape ; the inner prism 
is usually closed, but all three may be closed or open, leaving 
a continuous dentine area along the centre of the crown; the 
inner is usually the largest, the postero-external the smallest. 
