460 CHORDATA. 
In the lower jaw the teeth are all upon one bone, so that 
the only criterion for distinguishing the kinds of teeth is by 
their position relative to the upper teeth. 
An incisor tooth is hence a tooth borne by the pre- 
maxilla or by the mandible exactly opposite it. A canine 
tooth is a tooth borne by the maxilla immediately con- 
tiguous to the suture between premaxilla and maxilla, or 
by the mandible immediately opposite to and biting im- 
mediately in front of it. 
Succession or TeeTH —In the majority of mammals 
there is a modification of the polyphyodont arrangement of 
reptiles and the succession isreduced to two (diphyodont). The 
first series of teeth is known as the Zacteal or deciduous series, 
which are sooner or later replaced by absorption of their 
roots and the pushing-up of the permanent series from below. 
The incisors and canines usually correspond in number in 
each series, but the deciduous molars are not so numerous 
as the permanent ones. Thus there results a distinction 
between the permanent molars, the front ones only being 
preceded by deciduous molars. The former are known as 
premolars and the latter as molars proper. 
There are many exceptions to the diphyodont condition, 
and even in typical forms there is often a retardation in 
appearance of the hind-molars (cf wisdom-teeth) which 
simulates the beginning or the vestige of another series. 
A dental formula is often used as a symbol of the dentition of a 
mammal. 
The evolution of a dental formula may be illustrated as follows :— 
DENTAL FoRMULA OF MAN. 
. 2—2 . ITI 2-2 ooo! 
1. Lncisors —, canines —, premolars —, molars ”-— = 32. 
2-2 I-1t 2—2 3-3 
The four figures mean right and left half of upper and lower jaw. 
2. 2.3, 04, pm. 3, m 2. 
Here the names are represented by initials, and it is recognised that 
in the greater proportion the right and left half are similar ; and, lastly, 
a knowledge of simple arithmetic is assumed and the total is omitted. 
2123 
3: 2123" 
As the teeth are always quoted from in front backwards, the initials 
-are superfluous and a very short, compact symbol is the final product. 
As already indicated, the structure of the teeth and that of 
the limbs form the two most diagnostic features, so that the 
