MILK D6N7^ITI0N 



51 



teeth in which the grinding surface is raised into a series of two, 

 to many, tubercles sharper or blunter as the ease may be ; — sharper 

 and fewer at the same time in carnivorous and especially in 

 insectivorous types, more abundant in onmivorous animals. To 

 this form of tooth the term " bunodont " is applied. There is 

 uo doubt that this is the earliest type of tooth ; but whether the 

 fewer or the more cusped condition is the primitive one is a 

 question that is reserved for consideration at the end of the 

 present chapter. The other type of grinding tooth is known as 

 " lophodont." This is exemplified by such types as the I'erisso- 

 dactyla and Ungulates generally, and by the Eodents. The tooth 

 is traversed by ridges which have generally a transverse direction 

 to the long axis of the jaw in which the tooth lies. The ridges 



J'IG. 36. — ^lolar teeth of Aeeratherium platj/cepfmlimi. xj-. «i.1-ot.3, Molars ; «j/i, meta- 

 lopli ; p.l-pA, i^remolars ; ph, protoloph ; pis.f, parastyle fossa ; te, tetartocone. 

 (After Osborn. ) 



may be regarded as having been developed between tubercles 

 which they connect and whose distinctness as tubercles is 

 thereby destroyed. Lophodont teeth are only found in vegetable- 

 feeding animals. 



The special characteristics of the teeth of various groups of 

 animals wiU be considered further under the accounts of the 

 several orders of recent and fossil Mannnalia. 



A very general feature of the teeth of the ilammalia is what 

 is usually termed the diphyodont dentition. In the majority of 

 cases there are two sets of teeth developed, of which the first 

 lasts for a comparatively short time, and is termed im account of 

 its usual time of appearance the " milk dentition " ; this is 

 replaced later by the permanent dentition. In lower vertebrates 

 the teeth are replaced as worn away. There is not, howe\er, 

 so great an antithesis in this matter between the ]\Iammalia 



