II TRITUBERCULY AND SEXTUBERCULY 59 
lacks finality as a convincing proof of the tritubercular tooth as 
a primitive Ungulate tooth. 
Professor Osborn has ingeniously utilised certain deviations 
from the normal type of tooth structure (for the group) in favour 
of his strongly-urged opinions. If the stages of development 
have been as he suggests, a retrogression would naturally be in 
the inverse order; thus the “apparently ‘triconodont’ lower 
molar of Thylacinus” may be interpreted as a retrogression irom 
a tritubercular tooth. In the same way may be explained the 
triconodont teeth of Seals and of the Cetacean Zeuglodon. Finally, 
the modern Toothed Whales have retrograded into “ haplodonty.” 
Embryological evidence has also been called in, and with 
some success, to contribute towards the proof of the tritubercular 
theory of teeth. Taeker has shown that in the Horse and the 
Pig, and some other Ungulates, there is first of all a single 
hillock or cusp, and that later the additional cones arise separately. 
An apparent stumbling-bleck raised by these investigations 1s 
that it is not always the protocone or its equivalent in the upper 
jaw which arises first, as it obviously ought to do phylogenetically. 
This, however, is not a final argument in either direction. We 
know from plenty of examples that ontogenetic processes some- 
times do not correspond in their order with phylogenetic changes. 
Thus in the mammalian heart the ventricle divides before the 
auricle; and of course, phylogenetically, the reverse ought to 
occur, since a divided auricle precedes a divided ventricle. This 
method of development has, moreover, been interpreted otherwise. 
It has been held to signify that the complex teeth of mammals 
are indeed derived from simple cones but by the fusion of a 
munber of those cones. 
On the other hand there are the claims of the multituber- 
cular theory of the origin of mammalian teeth to be considered. 
The palaeontological evidence has been already, to some extent, 
utilised. The occurrence of such teeth among the possible fore- 
runners of mammals, and in some of the most primitive types of 
Mammalia, has been referred to. Sehor Ameghino dwells upon 
the sextubercular condition of many primitive mammals even 
belonging to the Eutheria. Ina recent communication! he attempts 
to identify six tubercles in the molars of types belonging to a 
fo 
1 «*On the Primitive Type of the Plexodont Molars of Mammals,” Proc. Zool. 
Soc. 1899, p. 555. 
