1896. ] MAMMALIAN DENTITION. 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 
mamials, 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 (Hyrax) or separated from the premolars 
by a wide diastema (Lapirus 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 Hquus, the posterior premolars 
have been so much enlarged that the anterior cheek-teeth became 
functionless and aborted. 
Osborn (32) on palzontological evidence regards the single 
pm. 1 as a persistent milk-tooth. 
The Molar Teeth of the Mole. 
The 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 that it is highly suggestive of a rudiment of a successional 
tooth (Plate XXVI. 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 m.1 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 (8) is 
present; the protocone (7) is small, whereas the paracone and 
metacone (5&6), 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 
