410 Mr. G. R. Waterhouse on the Classification of Mammalia. 



character which I do not recollect to have met with, except in the 

 Lemurida ; here it is sometimes of moderate size, hut generally 

 very small*. The resemblance in the dentition may be said to 

 extend to number and form, excepting that in Tupaia there is an 

 extra small false molar in the lower jaw : the lower incisors here 

 have moreover the same horizontal direction and the same little 

 keel along their upper sm'face as in the Lemurs. 



I will here make one or two general remarks upon dentition. On 

 the structure I will not comment ; but as regards the number of 

 certain teeth, some generalizations may be obtained which are im- 

 portant, and noneof which are violated by the arrangement adopted. 

 In the first place, in the Placental series there are never more than 

 six incisors in each jaw; this is what may be termed the normal 

 number in the Placentalia ; an occasional absence of the full 

 nmnber in some groups is unimportant, as nearly allied species 

 (in the Carnivora for instance) sometimes differ in the number 

 of the incisors, and even the same individual may, when young, 

 have the full number, but not when adult. There are cases, 

 however, in which there is a permanent reduction in the number 



4 



of incisors, as in the Quadrumana, which have normally -j. I 



can only call to mind two exceptions even in which there are 

 less than this number, and they are in Tarsius spectrum, where 



1 4 '^ 



there are—, and the Aye- Aye, where there are y- The Cheiro- 

 ptera have never more than fom' incisors in the upper jaw, and 

 it is only in the lower divisions of the group that that number 



is exceeded, there being — . In the Insectivora the incisors are 



sometimes clearly -^ and sometimes less, but in the greater por- 

 tion of the species the intermaxillary suture is obliterated at so 

 early an age, that the precise number of incisors has never been 

 determined. The Marsupiata are remarkable either for having 

 the incisors exceeding the normal number, being sometimes as 



many as — or —, or for having but two incisors in the lower 

 jaw when there are less than eight above ; no Marsupial has in- 

 cisors -Qj and there is but one species (the Wombat) in which 



the number in both jaws is the same. As regards true molars, 

 there is no case, among the Placentalia clearly made out in which 



3 3 ^ 



there are more than 3-::73. In the Marsupiata there are nor- 



4 — 4 



M^ally ^ _ ^ . The Carnivora (with one exception only) never 



* Often there is more than one of these minute perforations in the malar 

 bone of the Lemurs. 



