CHAP, x.] TERMINAL TEETH. 269 



hemidactylus No. 199 [compare with Ateles marginatus No. 200], 

 Phoca vitidina No. 336 ; and that on the other hand they mergf 

 into cases of supernumerary teeth standing outside or inside the 

 series, and whose forms do not correspond closely to those of 

 any tooth in the series, may be seen by comparison with Otaria 

 ursina No. 325, Phoca vitidina No. 329, Phalanger oriental No. 

 372. Though in some cases the shapes of duplicate teeth make a 

 near approach to the shapes of normal teeth, yet they are never 

 exactly the same in both, and teeth whose forms approach so 

 nearly to those of other teeth in the series as to suggest that they 

 are duplicates of them and that they may have arisen by multipli- 

 cation of the same germ, cannot be accurately distinguished from 

 extra teeth whose forms agree with none in the normal series. 



(5) Presence and Absence of Teeth standing at the ends of Series 

 (first premolars, last molars): the least size of particular TeetJi. 



Of the cases of numerical Variation in teeth the larger number 

 concern the presence or absence of teeth standing at the ends of 

 Series. As was mentioned in introducing the subject of dental 

 Variation, in many heterodont forms the teeth at the anterior end 

 of the series of premolars and molars are small teeth, standing to 

 the teeth behind them as the first terms of a series more or less 

 regularly progressing in size. Not only in teeth but in the case 

 of members standing in such a position in other series of organs, 

 e.g. digits, considerable frequency of Variation is usual. 



Variability at the ends of Series is manifested not only in the 

 frequency of cases of absence of terminal members, but also in the 

 frequency of cases of presence of an extra member in their neigh- 

 bourhood. An additional tooth in this region may appear in 

 several forms. It may be a clear duplicate, standing at the same 

 level as the first premolar (e.g. Cat, No. 270). On the other hand, 

 as seen in the Dogs (Nos. 232 and 233) there may be two teeth 

 standing between the canine and (in the Dog) the second pre- 

 molar. The various possibilities as to the homologies of the teeth 

 may then be thus expressed. The posterior of the two small teeth 

 may correspond with the normal first premolar, and the anterior may 

 be an extra tooth representing the first premolar of some possible 

 ancestor having five premolars; or, the first of the two premolars 

 may be the normal, and the second be intercalated (see No. 224) ; 

 or, both the two teeth may be the equivalent of the normal first 

 premolar ; lastly, neither of the two may be the precise equivalent 

 of any tooth in the form with four premolars, Of these possibili- 

 ties the first is that commonly supposed (HENSEL and others) to 

 most nearly represent the truth. But the condition seen in cases 

 where there is an extra tooth on one side only, as in the Dogs 

 figured (Fig. 42), strongly suggests that neither of the two teeth 

 strictly corresponds with the one of the other side. Seeing that in 

 such cases the single tooth of the one side stands often at the level 



