DENTITION. 723 
alive, but, as the limit of growth is reached, the residue of soft pulp tends 
to disappear. ; 
The two most marked characteristics of the teeth of Mammals are 
that they are typically Aeterodont—that is, different from one another in 
form and function—and that the succession is practically reduced to 
two sets, a condition described as diphyodont as contrasted with the 
polyphyodont condition seen in Fishes and Reptiles, where the suc- 
cession is practically unlimited. 
As exceptions, there are cases like that of the dolphins, where the 
teeth are uniform or. homodont and very numerous. This, however, 
is not a primitive but a secondarily acquired condition. 
In the typical dentition of Mammals there are forty-four 
permanent teeth, eleven on each side above and below; but 
it is rare in the Eutherian Mammals to find the full number 
developed, and the dentitions of the Marsupials, of the 
Edentates, and of the Cetacea cannot be reduced to this 
type. The eleven on each of the upper jaws may be divided 
in the typical case into four sets. Most anteriorly, associated 
with the premaxilla, are three simple, single-rooted teeth, 
usually adapted for cutting or seizing. These are called 
incisors. Posteriorly there are crushing or grinding teeth, 
whose crowns bear cusps or cones, or are variously ridged, 
and which have two or more roots associated with the 
maxilla. But of these grinders the last three occur as one 
set, having no calcified successors, or, as others maintain, 
having no milk predecessors. They are therefore dis- 
tinguished, as true molars, from.the four more anterior and 
often simpler premolars, which usually occur in two sets, 
the milk set being replaced by a permanent set. In many 
cases, however, the first premolar seems to be only once 
represented, Finally, the tooth just behind the incisors— 
that is to say, immediately posterior to the suture between 
premaxilla and maxilla—is distinguished as the canine, and 
is often long and sharp. 
This classification of teeth is in great part one of convenience: thus 
the distinction between incisors and grinding teeth is anatomical, that 
between molars and premolars refers to the history of these teeth ; the 
connection between the teeth and the subjacent bones is a secondary 
matter; there is often little to differentiate canine from premolar. 
Moreover, the teeth of the lower jaw, which is a single bone on each 
side, cannot be so certainly classified as those of the upper jaw. Here 
the lower canine is defined as the tooth which bites in front of the 
upper, and the incisors as the teeth in front of this tooth. 
