402 



MURID.E— 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 



(3) 



(4) 



Fig. 6i. — (i) Anterior Left Lower Cheek-Tooth of Arvkola amphibius, tilted so as 

 to show open pulp cavities. (2) LEFT LoWER Cheek-Teeth of Ewtomys glareolus, 

 both seen from outer side. (3) E. glareolus, simple form of m^, (4) E. siomerensis, 

 complex mK (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, m'- and m^ 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. 



