1896.] 



151 



[Farr. 



of wear of teetli, as Pm. 4 is usually more ■worn by attrition than Pm. B. 

 After the deciduous teeth are replaced by those of the permanent set, 

 M. 8 appears in both jaws. 



It is not possible to tell from available material whether the incisors 

 and canines are replaced or are persistent. In the later horse from the 

 Equus beds, the incisors were certainly replaced, and the germ of canine 

 is seen piercing the jaw. The foramen, through which it is growing, is 

 large, but it is not possible to determine whether it had a predecessor or 

 represents a permanent canine which does not appear until the other 

 teeth are developed. Chauveau* makes the statement that the canine 

 persists and is not replaced in the horse. However, judging from 

 analogy, we are quite safe in presuming that in M. huirdi both the in- 

 cisors and canines had predecessors in the milk series. 



The Milk Dentition. 



The temporary dentition may be given in the following formula : 

 I. f, C. \, D. f. See Fig. 1. 



The tooth which represents Pm. 1 of the adult skull is not a true milk 

 tooth, as it does not appear until the other teeth of the milk set are full}^ 

 developed, and is not replaced as are the teeth of the temporary series. 

 It may be considered a persistent 

 milk tooth, as it has no predeces- 

 sor, and then the dental formula 

 will be as given above. If consid- 

 ered one of the permanent set, as 

 there are ample reasons for doing, 

 the molar formula will be : D.f. 



The ditferences between the de- 

 ciduous teeth and those of the per- 

 manent set are not due to any ad- 

 dition or reduction in the number 

 of elements entering into the forma- 

 tion of the teeth, but are due to the 

 difl'erence in the relative develop- 

 ment of the elements in the two 

 sets. The differences can best be described by instituting a comparison 

 between the two sets, and to do this it will be best to describe those of 

 the permanent set and then show how the deciduous molars diti'er from 

 them. The last two of the temporary set ditfer only in minute detail 

 from the corresponding teeth of the later set, but there is a fundamental 

 difference between Pm. 2 of the permanent set and its predecessor in the 

 milk series. 



All of the premolar teeth, with the single exception of Pm. 1, are 

 molariform. Pm. 3 of both jaws presents some points of difference 



Milk Molars of M. bairdi, \. 

 a, superior series. 

 b, inferior series. 



* Comparative Anatomy of the Domesticated Animals. 



