574 UNGULATES. 



of dentine analogous to that in the leptorhine Rhinoceros (PI. 138, 

 fig. 7). The posterior crescentic enamel island (c) is a further 

 development of the fold (c) in the Rhinoceros' molar, but is much 

 earlier insulated in the Horse. 



In the lower jaw the same analogies may be traced : the teeth 

 here, as is usual in other quadrupeds, are narrower transversely 

 than in the upper jaw ; they are divided externally into two convex 

 lobes (PI. 136, fig. 2, m 1, o) by a median longitudinal fissure, 

 and on the inner side they present three principal unequal convex 

 ridges, and an anterior and posterior narrower ridge ; but the 

 crown of the molar is penetrated from the inner side by deeper and 

 more complex folds than in the Anoplothere and still more so than 

 in the Rhinoceros or Palceothere. The anterior valley between the 

 narrow ridge and first principal internal column expands into a 

 sub-crescentic fold : the second is a short simple fold and terminates 

 opposite that which penetrates the tooth from the outer side : the 

 third inner fold expands in the posterior lobe of the tooth like the 

 first ; two short folds partially detach a small accessory lobe at the 

 posterior part of the crown. All the valleys, fissures or folds in both 

 upper and lower molar teeth, are lined by enamel, which also coats 

 the whole exterior surface of the crown. 



The character by which the Horse's molars may best be dis- 

 tinguished from the teeth of other Herbivora corresponding with 

 them in size, is the great length of the tooth before it divides 

 into fangs. This division, indeed, does not begin to take place 

 until much of the crown has been worn away ; and thus, except 

 in old Horses, a considerable proportion of the whole of the molar 

 is implanted in the socket by an undivided base. This is slightly 

 curved in the upper molars, the outer side of the bases of which is 

 shown in PL 136, fig. 5, the inner side in fig. 6. The deciduous 

 molars have shorter bodies and sooner begin to develope roots, as 

 in fig. 5, cZ 2, 3 & 4 ; but in these, or in an old permanent molar 

 with roots, the pattern of the grinding surface, as it is shewn in 

 figs. 1 & 2, though it be a little changed by partial obliteration of 

 the enamel folds, yet generally retains as much of its character as 

 to serve, with the form of the tooth, to distinguish such tooth 

 from the rooted molar of a Ruminant. 



