- 
1896. ] MAMMALIAN DENTITION. ral 
ment of the lingually-placed dental Jamina would certainly suggest 
that 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 
9 - SI chart 
great enlargement, as is the case with 4 of the Shrew, it is 
invariably the permanent teeth which are enlarged, and not unfre- 
quently the corresponding milk-teeth are reduced and even 
aborted (Lepus &e.). This condition is so universal that I am 
inclined to believe that in the Shrew, in the case of es as with 
the rest of the incisors &c., the milk set has been reduced, but 
that here this 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 
whjch 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 has (1G. 4) (0 2) 3 4 (1 2 3 
lw. Jo (8) #). a! 
9 =a sa) — me (3 M.< ———— 
1 2 (8) LO 0.0 0 4 (Ut fe; acd 
CENTETES. 
My material for the study of this interesting form consisted of 
two foctal specimens of different ages, measuring respectively in 
total length 36 mm., head length 12 mm., and 70 mm. with ahead 
length of 20 mm., together with young and adult dried skulls in 
the teaching collection of the Royal 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 known, the most striking being the non-replacement 
of the 3rd upper incisor. This is especially interesting on account 
of what we have seen in Gymnura and Erinaceus, where that tooth 
is likewise only functional in one dentition ; but here the resem- 
plance seems to stop, for in Gymnura and Hrinaceus the functional 
third incisor undoubtedly belongs to the replacing ee (oe 
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