TOXODONTIA. 339 



worn down below the inflections of enamel, so that it has 

 lost its roughened grinding surface, and is reduced to a 

 smooth area of dentine. 



Not much is known of the structure of the dental tissues of 

 the Ungulata which calls for mention in an elementary work. 

 The thick cement of the crown of the teeth of the Horse, 

 and indeed of most of the group which possess thick cement, 

 contains many " encapsuled lacunse," and is developed from 

 a distinct cement organ of cartilaginous consistence (see 

 page 144). 



TOXODONTIA. 



The existing ungulate animals form only a small propor- 

 tion of those once peopling the earth, and many extinct forms 

 have been discovered, which while having affinities with the 

 Ungulata, can yet hardly be classified under any existing 

 order. For example, Toxodon, a creature equalling the 

 Hippopotamus in size, which was discovered by Mr. Darwin 

 in late tertiary deposits of South America, has a dentition 

 recalling in some respects the Bruta, in others the Rodents. 



It possessed in the upper jaw two pairs of incisors, the 

 median pair small, the outer exceedingly large, with per- 

 sistent pulps, and long curved sockets extending back to the 

 region of the molars, just as in existing Rodents. 



In the lower jaw there were three pairs of incisors, sub- 

 equal in size, and growing from persistent pulps ; they 

 resemble the incisors of Rodents in having a partial invest- 

 ment with enamel, but differ from them in being prismatic 

 in section, and in having the enamel disposed on two sides 

 of the prism. 



The molars were also very remarkable ; they grew from 

 persistent pulps, and had curved sockets, but the curvature 

 of these was in the reverse direction to that which obtains 



z 2 



