236 DORE. [ VoL. III. 
= 
below, have become specialized as sectorials, while the fifth, 
sixth, and seventh have, firstly, remained tubercular as in the 
dogs, or, secondly, have been lost, as in hyzenas and cats. . 
The reduction of the number of molars in relation to the in- 
crease in the size of the canines commenced as early as the 
Jurassic period.t It is seen in the genera Triconodon (Owen) 
and Paurodon (Marsh), where the canines are large and the molars 
few. In the Plagiaulacidz a similar relation is seen between 
the development of the incisors and the reduction in number of 
the molars. This is the modification of relation observed in ex- 
isting Mammalia of the orders Proboscidia and Rodentia, which 
will be mentioned later, under the head of proal dentition. 
8. ORIGIN OF THE AMBLYPODOUS DENTITION. 
As the Amblypoda form the only order of ungulate Mammalia 
with tritubercular superior and tuberculosectorial inferior mo- 
lars, the question has arisen in my mind why they did not develop 
a sectorial dentition in the same way, and for the same mechan- 
ical reasons, that the unguiculate series has done so. Having 
assigned certain mechanical reasons for the evolution of the 
sectorial teeth of the Carnivora, it is necessary to explain why 
the Amblypoda, which had apparently the same mechanical con- 
ditions at the start, did not eventually produce the same result. 
In the first place I observe in the families Coryphodontide 
and Uintatheriide of the Amblypoda, that the shearing of the 
inferior molar crests against the superior molar crests, is from 
before backwards. In the Creodonta and Carnivora it is from 
behind forwards. I supposed the latter movement to be due in 
these animals to the wedging of the inferior canine in front of 
the superior canine, a movement undoubtedly sufficient to ac- 
count for such a shearing, other things being equal. But in the 
Coryphodontidz the canines are greatly developed, yet the shear- 
ing of: the molar crests is in the opposite direction. It is also 
evident that the development of the canines cannot have been 
the cause of the maintenance of any kind of a shear between 
alternating parts of molar teeth, otherwise the quadritubercular 
1 My attention has been called to this point by my friend, Prof. H. F. Osborn, who 
has recently written fully on the Mesozoic mammals, and whose nomenclature of the 
dental cusps is here adopted. 
