274 PROBOSCIDIA. 



Two dental characters, however, exist, though hitherto 

 I believe unnoticed as such, which distinguish in a well- 

 marked and unequivocal manner, the genus Mastodon from 

 the genus Elephas. The first is the presence of two tusks 

 in the lower jaw of both sexes of the Mastodon, one or 



both of which are retained in the male, and acquire a 



^ 



sufficiently conspicuous size, though small in proportion to 

 the upper tusks ; while both are early shed in the female. 

 The second character is equally decisive ; it is the dis- 

 placement of the first and second molars in the vertical 

 direction, by a tooth of simpler form than the second, a 

 true dent de r emplacement, developed above the deciduous 

 teeth in the upper, and below them in the under jaw. 



These two dental characters, which are of greater im- 

 portance than many accepted by modern zoologists as 

 sufficient demarcations of existing generic groups of Mam- 

 malia, have been recognised in the species called Mas- 

 todon giganteus, most common in North America, and in 

 the Mastodon angustidens, which is the prevailing species 

 of Europe. 



To the last-named species I refer the comparatively few 

 remains of the Mastodon that have been discovered in Eng- 



ber of about ten, surrounded and held together by what Parkinson terms the 

 crusta petrosa. Now the enamel of the grinder of the Mastodon is all external, 

 whilst the crusta petrosa, or a substance resembling it, is internal." In fully- 

 formed and worn teeth of Mastodons, the dentine or substance which supports 

 the enamel degenerates, near the pulp cavity, into a kind of coarse bone-like 

 tissue approaching in structure to crusta petrosa, or cement ; but the same 

 tissue is found in the internal part of the dentine of the old grinders of the 

 Elephant. The truth is, that the exterior of the fangs in all Mastodons is co- 

 vered by a moderately thick coat of cement (cortical of Cuvier, crusta petrosa 

 of Clift); and that this substance extends upon the enamel of the crown, in a 

 very thin layer, requiring microscopical sections and examination for its detection 

 in the typical Mastodons ; but augmenting in thickness in the elephantoid and 

 other Mastodons, with thinner and more numerous transverse divisions of the 

 crown of the grinders. 



