1896.] MAMMALIAN DBNTITION. 571 



ment of the lingually-placed dental lamina would certainly suggest 

 tliat they should be regarded as persistent milk-teeth ; but against 

 this view we have the fact that all the other anterior teeth (incisors, 

 canine, and premolars) are now shown to be permanent teeth with 

 vestigial milk predecessors, and also that in all other cases among 

 the Placentalia where the teeth (especially the incisors) undergo 



great enlargement, as is the case with ~ of the Shrew, it is 



invariably the permanent teeth which are enlarged, and notunfre- 

 quently the corresponding milk-teeth are reduced and even 

 aborted (Lepiis Ac). This condition is so universal that I am 



inclined to believe that in the Shrew, in the case of r^ as with 



the rest of the incisors &c., the milk set has been reduced,' but 

 that here tliis reduction has been carried further until all trace of 

 di. 1 has been lost, this being due to the large size and earlier 

 development of pi. 1, these latter being developed far in advance 

 of the posterior teeth. The lingual growth of the dental lamina 

 is comparable to that which has been observed in connection 

 with the successional teeth in so many forms (Seal (6), Dog (24), 

 &c.), and which is there regarded as evidence of the existence of a 

 3rd or 4th set of teeth which might replace the permanent set, and to 

 which the term postpermanent dentition has been applied. This 

 structure may owe its greater development in the Shrew to the 

 early appearance of the permanent set and to the complete loss 

 of the milk series. 



The relations of these teeth may be expressed as follows, 

 bearing in mind that the milk-dentition is functionless and 

 probably uncalcified : — 



fl 2 3 ri(i-4) (^ 2(c.) 3 4 fl 2 3 



1.\JS1JIK 0.\^S^; P.< aj|, Ji.l 



I 



\l 2 (5) L.0 'v.0 4 |_1 2 3 



Centbtes. 



My material for the study of this interesting form consisted of 

 two foetal specimens of different ages, measuring respectively in 

 total length 36 mm., head length 12 mm., and 70 mm. with a head 

 length of 20 mm., together with young and adult dried skulls in 

 the teaching collection of the Eoyal College of Science and the 

 more numerous specimens in the British Museum. 



The relations of the milk and permanent teeth of the Tanrec 

 are fairly well jtnown, the most striking being the non-replacement 

 of the 3rd upper incisor. This is especially interesting on account 

 of what we have seen in Oymnura and Erinaceus, where that tooth 

 is likewise only functional in one dentition ; but here the resem- 

 blance seems to stop, for in Gymnwa and Erinaceru the functional 

 third incisor undoubtedly belongs to the replacing or permanent 



37* 



