20 GENERAL ANATOMICAL CHARACTERS 
by others. These animals are called Monophyodont. But in the 
larger number of mammals, certain of the teeth are preceded by 
others, which may be only of a very transient, rudimentary, and 
functionless character (being in the Seals, for example, shed either 
before or within a few days after birth), or may be considerably 
developed, and functionally occupy the place of the permanent teeth 
for a somewhat lengthened period, during the growth and develop- 
ment of the latter and of the jaws. In all cases these teeth 
disappear (by the absorption of their roots and shedding of the 
crowns) before the frame of the animal has acquired complete 
maturity, as evidenced by the coalescence of the epiphyses of the 
osseous system. As these teeth are, as a general rule, present 
during the period in which the animal is nourished by the milk of 
the mother, the name of “milk-teeth” (French dents de lait, 
German milchzéhne) has been commonly accorded to them, although 
it must be understood that the epoch of their presence is by no 
means necessarily synchronous with that of lactation. Animals 
possessing such teeth are called Diphyodont. No mammal is known 
to have more than two sets of teeth ; and the definite and orderly 
replacement of certain members of the series is a process of quite a 
different nature from the indefinite succession which takes place in 
all the teeth continuously throughout the lifetime of the lower 
vertebrates. 
When the milk-teeth are well developed, and continue in place 
during the greater part of the animal’s growth, as is especially the 
case with the Ungulata, and, though to a less degree, with the 
Primates and Carnivora, their use is obvious, since taken all together 
they form structurally a complete epitome on a small scale of the 
more numerous and larger permanent set (see Fig. 3), and, con- 
sequently, are able to perform the same functions, while time is 
allowed for the gradual maturation of the latter, and especially 
while the jaws of the growing animal are acquiring the size and 
strength sufficient to support the permanent teeth. Those animals, 
therefore, that have a well-developed and tolerably persistent set of 
milk-teeth may be considered to be in a higher state of development, 
as regards their dentition, than those that have the milk-teeth 
absent or rudimentary. 
It is a very general rule that individual teeth of the milk and 
permanent set have a close relationship to one another, being 
originally formed,as mentioned above, in exceedingly near proximity, 
and with, at all events so far as the enamel-germ is concerned, a 
direct connection. Moreover, since the latter ultimately come to 
occupy the position in the alveolar border temporarily held by the 
former, they are spoken of respectively as the predecessors or suc- 
cessors of each other. But it must be understood that milk-teeth 
may be present which have no successors in the permanent series, 
