454 MAMMALIAN ANATOMY 



are all modifications of one type. It would appear as if the region of 

 the jaw of greatest functional importance were moving forward to a 

 point just anterior to the sectorial teeth. It is possible that these 

 variations in the upper premolar may occur only in short-faced cats 

 and are the result of domestication, which has removed the necessity 

 of the very active use of the canines and hence has reduced the length 

 of the muzzle and the gape of the mouth. That some such influence 

 is at work is shown by the occasional absence of the diastemata on 

 either side of the small first upper premolar. In young kittens the 

 face is relatively much shorter than in adults and the sectorial teeth 

 are relatively further forward, since, as we have seen (page 424), the 

 permanent second premolar replaces the deciduous sectorial. 



When the lower teeth close within the upper teeth (Fig. 371) the 

 lower second premolar shears against the upper second premolar (Fig. 

 383) ; hence the former exhibits an area of wear on the outer anterior 

 surface of the principal cusp (Fig. 379, x), and the upper premolar 

 exhibits a corresponding area on the posterior inner part of its 

 principal cusp. This grinding away of the surface appears always to 

 begin on the cingulum (Fig. 372, x) at a point marked by a swelling 

 or minute cusplet where the apex of the lower premolar strikes with 

 greatest force ; thence it gradually extends downward on the crown 

 (Figs. 374, 375). In many specimens of this tooth the outer surface 

 of the posterior fang is marked by a vertical groove which begins 

 below, just above the point where the lower premolar strikes, and is 

 continued upward to the tip of the fang (Fig. 370). In some 

 specimens this groove is shallow, in others so deep that two coalesced 

 posterior fangs are produced (Fig. 380). The inner cusplet of the 

 cingulum is then more conspicuous. In the next stage of this varia- 

 tion the two posterior fangs are separate (Figs. 377, 378, 381), and the 

 new fang is on the inner side, almost directly over the middle of the 

 tooth and crowned by a very distinct cusp. The middle fang gradually 

 diverges from the posterior fang (Fig. 378) and is directed upward 

 and forward. This divergence I believe to be due to the pressure 

 exerted by the lower tooth, inasmuch as the line of divergence lies in 

 the arc of the circle described by the tip of the lower tooth in the 

 vertical movements of the lower jaw. 



The development of the inner cusp, or deuterocone, and the sepa- 

 ration of the inner fang are accompanied by a general increase in the 



