108 



MR. W. BATESON ON 



[Feb. 



which is thus partially divided into two. Upon comparing this with 

 the series on the left side, it appears that two of the six teeth stand 

 level with this bigeminous tooth, and both bite between p"' and mi 

 of tlie lower series, though there is no crowding in the jaw owing 

 to the large spaces which exist between the normal teeth of this 

 species. There is, therefore, no doubt that the fourth premolar is 

 on the left side bodily represented by two distinct teeth, each of 



Fig. 2. 



Phoca grcenhnulica, having the fourth upper premolar on the right side bigemi- 

 nous, and on the left side represented by two complete teeth (shaded). 

 Eight and left profiles ; from a specimen in the Leyden Museum. 



which is perfect and complete, while on the right side the process of 

 division is incomplete. 



In the Paris Museum (A 2897) there is a specimen of P. grcen- 

 landica in which the second upper right premolar is represented by 

 two teeth, each of which has two roots ; but these two teeth stand 

 at the same level in the arcade, one being external and the other 

 internal. On the left side the second upper premolar is incompletely 

 double, the crown being partially divided by an oblique constriction 

 into an anterior and internal portion and a larger posterior and ex- 

 ternal part. The former has one root and the latter has two. 



It is thus seen that reduplication of teeth may occur in such a 

 way that a tooth which is normally single may be represented by 

 two teeth, and that the two teeth thus formed may either (I) both 

 take places in the ordinary series, or (2) may stand externally and 

 internally respectively. 



Substitution of two teeth for one, both being in series, was seen 

 in Ommatophoca rossii (p^), Phoca groenlandica (p^), Otaria ursina 

 (P^), Brachyteles hemidactylus (p'), Phalanger orientalis (p^), ditto 

 (p^), Myrmecobius fasciatus (J^j, ditto (i*). Probably also cases of 

 the presence of two similar small teeth in place of the first premolar 

 in the Canidae and Felidse should be looked on as belonging to this 



