O74 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 remplacement, 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 A/as- 
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 
erusta 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, ina 
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, 
