RHINOCEROS. 593 



surface, and two others continued from this obliquely backwards 

 to the inner border, one from the antero-external angle, the other 

 from near the middle of the outer wall ; the inner terminations of these 

 two parallel oblique ridges form the summits of the two cones which 

 constitute the inner half of the crown. (1) Small or secondary ridges pro- 

 ject from the sides of the principal ridges into the intervening valleys, 

 in extent and number varying according to the species, the most 

 constant being the one, marked / in fig. 5, from the posterior oblique 

 ridge. The first effect of mastication is to wear away the enamel from 

 the summits of the ridges, and to expose a tract of dentine which widens 

 as attrition proceeds, varying the pattern of the grinding surface as 

 the valleys are thus progressively obliterated. These changes are 

 illustrated by the figure of the molar series of the upper jaw of the 

 Rhinoceros indicus (PI. 138, fig. 3). The chief valley, marked b in fig. 5, 

 is expanded and bilobed at its termination, and is deepest at each 

 terminal division, and at the middle of its course; in the second 

 true molar (in 2) its entire extent is shown ; in the last molar (m 3) 

 the posterior terminal division is insulated by the wearing away of 

 the enamel from the shallower part of the valley between the two 

 divisions : the same insulation has taken place in m 1 , and the shallow 

 entry of the valley is almost worn away ; in p 4 it is obliterated, 

 and the peninsular fold of enamel is converted into an island. 

 The same change is eff'ected in the short posterior fold c, it deepens 

 as it penetrates the crown, and in w 4 its beginning has been worn 

 down to the dentine, and its end converted into an island of enamel. 

 The same three islands are show^n in ^ 3 and p 2, with the addition 

 of linear tracts formed by the wearing down of the crown to the 

 basal ridge at the antero-internal lobe. The grinding surface is 

 sometimes reduced to a plain tract of dentine before the molar is 

 shed. There is no posterior fold c in the last molar. (2) 



The modifications in the form of the valleys and secondary 



(1) See the germ of the upper molar, PI. 138, fig. 8. 



(2) These, and similar details in other chapters of this work, may be deemed tediously, 

 perhaps unnecessarily, minute : they are, however, indispensable to whoever would make suc- 

 cessfully the noble application of anatomy to the restitution of lost species and the past history 

 of the globe. 



Q Q 



