1248 



CLASS MAMMALIA. 



retarded, and their function filled for a time by an earlier series 

 of so-called milk-teeth ; such Mammals being accordingly termed 

 Diphyodont. As development proceeds the permanent teeth in such 

 Mammals come up beneath the milk-teeth, and thus replace them 

 in a vertical direction ; but there are instances where certain of these 

 milk-teeth have no such permanent successors, while in other cases 

 the anterior teeth which come into use with the permanent denti- 

 tion have no milk predecessors. Those Mammals with a Mono- 



Fig. 1127. — Outer lateral aspect of the left dentition of the Pig (Sus scrofa), with the outer 

 lamina of bone removed, in order to exhibit the roots of the teeth, z, Incisors ; c, Canine ; fim t 

 Premolars ; m, True molars. 



phyodont dentition present the least specialised development, since 

 the milk-series appears from the latest researches to be an addition 

 grafted on to the permanent one; and there is accordingly no homo- 

 logy between this definite single replacement and the irregular con- 

 tinuous change which takes place in many Reptiles. In some Mam- 

 mals, like the Dolphins (Delphinidce), all the teeth are so much alike 

 that they cannot be divided into groups, and the dentition is then 

 described as Homoeodont ; but in the majority the permanent teeth, 

 either from their position or their mode of succession, can be sepa- 

 rated into four distinct groups (as in figs. 112 6, 1127), and the den- 

 tition is then termed Heterodont. In Eutherian Diphyodont Mam- 

 mals the total number of teeth of the permanent series does not 

 normally exceed 44; and in forms like Sus (fig. 1127), or the ex- 

 tinct Anoplotherium, where this full complement is present, the first 

 three upper teeth (/ 1-/3) on either side, which are situated in the 

 premaxillse, are termed incisors ; the last three (m \-m 3), which are 

 distinguished by having no milk-predecessors, true molars ; the four 

 submolariform teeth (pm i-pm 4) in advance of the latter, of which 

 the last three have such deciduous predecessors, premolars : and 



