DENTAL SYSTEM 23 
ported on two or more roots. They crush or grind the food, and 
are hence called “molars.” Many animals have, between these 
two sets, a tooth at each corner of the mouth, longer and more 
pointed than the others, adapted for tearing or stabbing, or for 
fixing struggling prey. From the conspicuous development of 
such teeth in the Carnivora, especially the Dogs, they have received 
the name of “canines.” A dentition with its component parts so 
differently formed that these distinctive terms are applicable to 
them is called Heterodont. In most cases, though by no means 
invariably, animals with Heterodont dentition are also Diphyodont. 
This general arrangement is extremely obvious in a considerable 
number of mammals; and closer examination shows that, under 
very great modification in detail, there is a remarkable uniformity 
of essential characters in the dentition of a large number of 
members of the class belonging to different orders and not otherwise 
closely allied ; so much so indeed that it has been possible (chiefly 
through the researches of Sir Richard Owen) to formulate a common 
plan of dentition from which the others have been derived by the 
alteration of some and suppression of other members of the series, 
and occasionally, but very rarely, by addition. The records of 
paleontology fully confirm this view, as by tracing back many 
groups now widely separated in dental characters we find a 
gradual approximation toa common type. In this generalised form 
of mammalian dentition (which is best exemplified in the genera 
Anoplotherium and Homalodontotheriwm) the entire number of teeth 
present is 44, or 11 above and 11 below on each side. Those of 
each jaw are placed in continuous series without intervals between 
them; and, although the anterior teeth are simple and _ single- 
rooted, and the posterior teeth complex and with several roots, 
the transition between the two kinds is gradual. 
In dividing and grouping such teeth for the purpose of descrip- 
tion and comparison, more definite characters are required than 
those derived merely from form or function. The first step towards 
a classification has been made by the observation that the upper 
jaw is composed of two bones, the premaxilla and the maxilla, 
and that the suture between these bones separates the three 
anterior teeth from the others. These three teeth, then, which are 
implanted by their roots in the premaxilla, form a distinct group, 
to which the name of “incisor” is applied. This distinction is, 
however, not so important as it appears at first sight, for, as 
mentioned when speaking of the development of the teeth, their 
connection with the bone is only of a secondary nature, and, although 
it happens conveniently for our purpose that in the great majority 
of cases the segmentation of the bone coincides with the interspace 
between the third and fourth tooth of the series, still, when it does 
not happen to do so, as in the case of the Mole, we must not give 
