TEETH OF UNGULATE. 347 



replace the abraded extremities. The direction of the abraded 

 surface is in part provided for by the partial disposition of the 

 enamel. The molar series consists of — 



The first premolar is small, far in advance of the second, and is 

 soon shed: the others (fig. 276, 2, 3, 4) form a continuous series 

 with the true molars (m, 2, 3). These have the double trefoil 

 character shown in fig. 272. The crown of the last, in the lower 

 jaw, is lengthened by a fifth cusp developed behind the normal 

 pairs. The large tusks, fig. 276, c, exhibit the maximum of density 

 in their component tissues. The enamel ' strikes fire ' with steel 

 like flint. The compact dentine has a high commercial value, 

 especially for the fabrication of artificial teeth. It differs from 

 true ivory by showing, in transverse section, the simple concentric 

 instead of the t engine-turned' or curvilinear decussating lines. 1 



The affinities of the Hippopotamus are clearly manifested by 

 the character of its deciduous dentition ; and if this be compared 

 with the dentition at a like immature period in other Ungulata, 

 it will be seen, by its closer correspondence with that of Artio- 

 dactyles, and more especially the Phacochere, that the Hippo- 

 potamus is essentially a gigantic Hog. 



The formula of the teeth which are shed and replaced, is — 



.2.2 1.1 3.3 '"* 

 2 272 ;c iTi ;W 3T3 = 24 - 



If the small and simple tooth, which is developed anterior to the 

 deciduous molars, and which has no successor, be regarded, from 

 its early loss in the existing Hippopotamus, as the first of the 

 deciduous series, we must then reckon with Cuvier four milk- 

 molars on each side of both jaws. 



The incisors in both jaws are simply conical and subequal, with 

 an entire cap of enamel on the crown. The deciduous canines 

 scarcely surpass them in size in the upper jaw, and not at all in 

 the lower. Projecting forward, here, from the angles of the 

 broad and straight symphysis, they appear like an additional pair 

 of incisors ; and this character of equality of development was 

 retained by the ancient form of Hippopotamus with the more 

 typical number of incisors, -f :f , which formerly inhabited India. 



The first true deciduous molar, d 2, has a conical crown and two 

 fangs in both jaws. That above has also a conical crown with 

 one strong posterior and two anterior ridges. The second 



1 In v. is described (p. 569) and figured (pi. 142), the lower tusk of a Hippopota- 

 mus which, after fracture, had been united again by a mass of ' osteodentine.' 



