542 



A very similar histological appearance is to be observed in the 

 Pig. It will be noticed how much more strongly the downgrowth on 

 the lingual side of the developing tooth, is marked than on the labial 

 side, from which it is possible to infer that when a successional does 

 develope, as is sometimes the case in this animal, it does so from 

 the lingual downgrowth, that is from the one which I regard as be- 

 longing to the post-permanent series. 



I have not as yet had an opportunity of fully examining the con- 

 dition as seen in the Hyrax. 



The view has been held that when pm^ is functional in one den- 

 tition only, it is a very much retarded milk tooth. If such were the 

 case, we should then have to regard the labial growth seen in con- 

 nection with the dental lamina of this tooth, and this tooth alone, as 

 a structure referable to the pre-milk dentition. 



Now I think it will generally be admitted that^m^ is undergoing 

 suppression in the Dog^) as it has already done in the Aeluroids. It 

 would then be necessary to assume that, with the gradual suppression 

 of this tooth there is a retention of its representative in the pre-milk 

 series. It seems to me, however, unlikely that such a vestigial struc- 

 ture would be, under the circumstances, preserved, and I prefer to 

 believe that this tooth is to be regarded as belonging to the perma- 

 nent series and that its milk predecessor has been suppressed leaving 

 only traces behind. 



The other functional teeth which are present in one dentition only 

 are the molars. Here again two views have been held by difierent 

 writers; one being, that they are delayed milk teeth; the other, that 

 they belong to the permanent series. 



We find in the Dog only one functional set of teeth and a well- 

 marked downgrowth of the dental lamina on the lingual side, possibly 

 representing a second dentition. No evidence of any downgrowth on 

 the labial side is to be seen, unless it disappears before birth, for I 

 have not examined toetal pups. In any case its presence would only 

 strengthen my contention. 



If the previous conclusions with regard to pm^ be accepted, then 

 I think we must also conclude that the molars belong to the per- 

 manent series. For, if such be not the case, then we have a sup- 

 pression of a milk tooth at one end of the cheek teeth, and of a 

 permanent tooth, its milk predecessor still persisting, at the other. 

 This conclusion coincides with that of Woodward in the case of 



1) In one case which I dissected, after clarifying with oil of cloves 

 J"»^ was entirely wanting an one side. 



