34 PACHYDERMATA. 



outline, somewhat higher in front than behind, the dimensions 

 being about seven inches in length, by four of width at the anterior 

 end. The height of the crown, from the base of the enamel to 

 the summit of the first ridge (vide Section, pi. 3, fig. 9), is about 

 two and a half inches, decreasing a little successively to the fourth 

 or last principal ridge. The grinding surface is divided in two 

 directions, viz., longitudinally along the axis by a narrow cleft, 

 which, as in the Hippopotamus and most other Pachydermata 

 bisects it into nearly equal segments ; and transversely by four 

 deep open hollows, alternating with as many trenchant ridges. 

 Each of the lateral divisions of these ridges is composed of a pair 

 of compressed confluent points ; in the upper molars the outer 

 division is more elevated, and slopes gradually towards the inner 

 side which, although lower, is the most complex in composition, 

 giving off the ' talons ' and accessory tubercles, which are more 

 or less developed in different species, the inner points being com- 

 monly the largest. This relation continues during the wear of 

 the teeth, the inner side being more worn by the process of tritura- 

 tion than the outer. The teeth of the lower jaw agree with the 

 upper in the form and subdivision of the crown ; but the dis- 

 position of the lateral segments is reversed, the inner being higher, 

 and continuing so during the wear, while the outer are lower, but 

 more complex in composition. The higher and lower portions 

 of the crowns of the opposed teeth are thus brought to act against 

 each other, and serve more effectually the triturating function of 

 the teeth. The plane of the grinding surface is nearly level from 

 back to front, both in the upper and inferior grinders. In conse- 

 quence of the peculiar mode of protrusion of the teeth, from 

 behind forwards, the crown ridges come successively into use, 

 commencing with the first, and each of the lateral divisions is 

 ground down to a rhomboidal disc, surrounded by a band of thick 

 enamel : as the wear of the tooth advances, the separate discs 

 unite, forming a wide transverse depression corresponding to the 

 configuration of the ivory nucleus, and the shell of enamel which 

 invests it. The 'cement,' or third dental substance, is very 

 sparingly developed upon the crowns of the molars of the 



