14 RE VISIOX OF AMEBIC AX MOLES— TR UE. 



of the sutuie is sucli that if used exclusively in determining the homol- 

 ogy of tlie teeth, as already stated, very different formula? from 

 those ordinarily accepted are obtained. Accepting Professor Flower's 

 dictum, what reason is there for ignoring the jwsitiou of the suture? 

 The reason is this: It is a well-known fact that in most placental mam- 

 mals the tirst premolar is without a milk i)redecessor. If, therefore, 

 we liud a tooth close to the intermaxillary suture, which is without a 

 predecessor, we may conclude that this tooth is the first premolar. 

 We may expect to find also a corresponding tooth in the lower jaw. 



In examining carefully the young skulls already nu'utioued, I find 

 this first premolar in many cases without difficulty and with certainty, 

 and I will now proceed to give conjointly the formuke obtained by 

 employing the two methods of determination. 



For jScapanus we have the following: 



Formnhi by premol.ar : i, ?, ; c, \ ; pm, ^ ; m, §. 

 Formula by suture: i, |; c, -}; pm, | ; m, f. 



In this genus, as will be observed, the intermaxillary suture comes 

 between the second and third anterior teeth, and if employed in iden- 

 tifying the teeth, gives the genus but two incisors on each side. The 

 fornuila T)ased on the first premolar is more in harmony with general 

 considerations.^ 



The genus Parascalops gives exactly the same results as ScapanuSy 

 though the material in this case is hardly as satisfactory. 



The genus Scalops gives the following results: 



Formula by premolar : i, f ; c, ■}', pm, j| ; m, ^. 

 Formuhi by suture : i, i ; f, j ; pm, f ; m, |. 



In this genus there is no difference in the formula whether based on 

 the position of the suture or on the premolar. A more important mat- 

 ter here is the question of the presence of a third lower incisor and 

 lower canine. The formula as given by Mivart, Dobson, and others is 

 i, ?, ; c, ^. But I find that in young skulls there are two small teeth 

 behind the second lower incisor, and what is considered to be the first 

 premolar. While these are like milk teeth in their simple form, their 

 position would api)ear to indicate that they do not belong to the milk 

 dentition, but to the permanent dentition. From the reduction of the 

 jaw they have become minute, and disappear before full growth is 

 attained. In the discussion of individual variation in the genus Seal- 

 ops (p. .'>7) 1 shall show that youngish skulls of this genus often present 

 at least a rudiment of the i)osterior of these two minute teeth and 

 occasionally a Avell developed tooth. It appears i)robable, therefore, 

 that in the lower jaw a reduction of the third and fourth teeth has 

 taken place, similar to that which has affected the second and third 

 teeth of the upper jaw, but the process has been carried further. It 



1 ScapaniiN (nitlioin/i is here left out of consideration, as it is based on a single adult 

 specimen. 



