252 MAMMALIAN DENTITION 



also intermediate in appearance. The lower first postcanine 

 exhibits merely a cingulum, no heel ; the second has a crown 

 approximating in appearance but by no means reproducing 

 the features of its successional premolar ; the molarif orm third 

 is longer and narroAver with a relatively larger trigonid and 

 smaller talonid than the first permanent molar. Analogous 

 differences exist in the upper teeth. The first has only a low 

 cingular cusp on the palatal side of the apex; in the second 

 which is elongated transversely this palatal cusp has increased 

 in size and prominence; in the third it forms a rounded pro- 

 tocone, the main cusp (paracone) is replaced by a reduplicated 

 one (paracone and metacone) and there is developed a small 

 hypocone. We note then that close resemblances between milk 

 teeth and their successors are accidental in nature and more to 

 be wondered at than striking differences.* 



When Ave turn our attention to other animals, Ave again con- 

 stantly find evidence of mingled ancestral and adaptive fea- 

 tures in the milk dentition. In the Aye-aye for example the 

 milk teeth are much less specialized than the permanent set 

 and clearly indicate by their more primitiA r e features the lemu- 

 roid character of the animal. In its milk dentition the Horse 

 possesses shorter croAvned teeth Avith characters simpler than 

 those of their complex, hypsodont, permanent successors. In 

 the milk dentition of the Dog (Fig. 93) there are three postca- 

 nines of Avhich the first upper and first and second lower are 

 simple premolariform teeth, the second upper and the third 

 loAver sectorial and the third upper molar if orm, thus reproduc- 

 ing in miniature the essential features of the adult dentition 

 as a ivhole. 



In carniA^orous animals generally the milk teeth play their 

 part and are shed early. In the Seal they are shed before birth. 

 In some animals, the Edentates, for example, the milk denti- 

 tion is quite A^estigial but such teeth as there are show much 

 less specialization than their permanent successors. The milk 



*The gradual increase in complexity of the milk molars backwards, representing as it 

 were stages in development culminating in the pattern of the first permanent molar is 

 very striking in the New-World Monkeys. The reader is referred in this connection 

 to statements made on pp. 62, 103 regarding the Premolar Analogy Theory. 



