1896.] MAMMAilAN DBNTITION. 579 



correct, in which case the above objection would not hold, and the 

 non-replaced pm. 1 may be regarded in all cases as a persistent 

 milk-tooth. , 



The presence or absence of the 1st premolar appears to be inti- 

 mately connected with the development of the canine, for in 

 mammals, other than the Insectivora, it is commonly wanting or 

 much reduced in all those forms possessed of a large canine tooth, 

 while in those forms in which it is present in both dentitions the 

 canine is either vestigial (Ifyrax) or separated from the premolars 

 by a wide diastema (JVapirus indicus). In the case with no suc- 

 cession to pm. 1, I should imagine that enlarged deciduous canine 

 caused a slight decrease in size of dpm. 1, while the enormous 

 permanent canine, which always develops early, caused a total 

 suppression of ppm. 1 ; on the other hand, in those cases where 

 pm. 1 is replaced, the non-development of the canines or their 

 early removal forward allows the germ of ppm. 1 to mature 

 and become functional. In forms such as the Pecora, in which 

 both the canine and pm. 1 are wanting, this latter tooth was 

 probably suppressed in some ancestor in whom the canine was well 

 developed, and probably all trace of its germ has been lost, so that 

 the subsequent loss of the canine has not caused pm. 1 to reappear ; 

 besides in these forms, as also in Equus, the posterior premolars 

 have been so much enlarged that the anterior cheek-teeth became 

 f unctionless aud aborted. 



Osborn (32) on palaeontological evidence regards the single 

 pm. 1 as a persistent milk-tooth. 



The Molar Teeth of the Mole. 

 Tlie lingual development of the dental lamina in relation to —^ 



is most conspicuous, it being more strongly developed in the Mole 

 and Centetes than in any other animals I have examined, so much 

 so tliat it is highly suggestive of a rudiment of a successional 

 tooth (Plate XXVL fig. 32, d.l.) ; a similar but slighter growth 

 is found in relation to m. 2. 



The Cusps. 



The molar teeth belong to the trituberculo-sectorial order; in the 

 lower molars the heel is very large and bears two strong cusps ; 

 the heel in mTl is larger than the trigon, but in m. 2 and m. 3 it is 

 smaller ; in all the protoconid is the largest and the paraconid the 

 smallest of the main cusps ; a small posterior cingulum-cusp is seen 

 in m. 1, while m. 2 bears in addition a similar anterior cusp, in m. 3 

 the anterior one alone is present. The upper molars (Plate XXVI. 

 fig. 35) are mainly tritubercular, but a very small hypocone (s) is 

 present ; the protocone (7) is small, whereas the paracone and 

 metaeone (6 & 0), especially the latter, are very large and show a 

 tendency to become crescentic or V-shaped, the summit of the 

 cone being situated some distance from the outer border of the 



