318 A MANUAL OF DENTAL ANATOMY. 



longitudinal ridge, possibly the ciugulum, and the transverse 

 ridges become oblique ; consequently the valley between 

 the ridges c and d is also oblique in direction, and a 

 second valley " a " behind the posterior ridge is introduced 

 (Fig. 134). 



The simplicity of the pattern is also departed from by 

 the margins of the ridges, and therefore the boundaries of 

 the depressions, being waved and irregular. 



The lower molars of the Rhinoceros are made up of two 

 crescentic ridges, one in front of the other, with the hollows 

 turned inwards. It is less obvious how this pattern is 

 derived from that of the Tapir, but it may be that the trans- 

 verse ridges of the Tapir type of tooth may have become 

 curved and crescentic, so that the original outer edge of the 

 posterior ridge abuts against the exterior of the ridge in 

 front of it. The valleys between the processes of enamel 

 and dentine of the tooth of the Rhinoceros, termed " sinuses,' 7 

 are not filled up solidly with cementum. The more complex 

 pattern which characterises the molar of the Horse may be 

 derived from a further modification of the Rhinoceros molar. 



To use the words of Professor Huxley : " Deepen the 

 valley, increase the curvature of the (outer) wall and laminae 

 (transverse ridges), give the latter a more directly backward 

 slope ; cause them to develop accessory ridges and pillars ; 

 and the upper molar of the Tapir will pass through the 

 structure of that of the Rhinoceros to that of the Horse." 



By a further increase in the obliquity of the ridges and 

 in their curvature (c and d), they become parallel to the 

 external or antero-posterior ridge (wall), and bend round 

 until they again touch it, thus arching round and completely 

 encircling the sinuses (a and the space between c and d) in 

 the Rhinoceros tooth. In this way the unsymmetrical pattern 

 of the Rhinoceros tooth may be supposed to become trans- 

 formed into the comparatively symmetrical one of the Horse 

 or of the ruminant. 



