MILK DKN riTTOX 



51 



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

 to many, tubercles sharper or blunter as tlie case may be ; — sharper 

 and fewer at the same time in carnivorous and especially in 

 insectivorous types, more abundant in omnivorous animals. Tu 

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

 no doubt that this is the earliest type of tooth ; but whether tbe 

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

 (juestion that is reserved for consideration at the end of the 

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

 " lophodont." This is exemplihed by such types as the Perisso- 

 dactyla and Ungulates generally, and ])y the Rodents. The tooth 

 is traversed l)y ridges which have generally a transverse direction 

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



i'lG. 36. — M^olar teetti of A rflmtheriMinplatijcepJialn))!. x!^. w?.l-'/«.3. Molars ; vi.h,meta.- 

 loph ; ji.l-pA. ])reiiiolars ; ph, protoloph ; j«;/', parastyle fossa ; te, tetartocoiie. 

 (After Osboni. ) 



may he regarded as liaving been developed l;)etween 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 will be considered further under the accounts of the 

 several orders of recent and fossil Mammalia. 



A very general feature of the teeth of the Mammalia is wliat 

 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 com])aratively short time, and is termed on account of 

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

 replaced later by the pernument dentition. In lower vertebrates 

 the teeth are replaced as worn away. There is not, however, 

 so t»:reat an antithesis in this matter between the Manunalia 



