1897.] Trituberculy : 999 
are found among the Mesozoic mammalia and have already 
be2n worked out with considerable care. 
From each of these great primary stages it would at first 
appear that some of the mammalia directly derived their den- 
tal type, for both the “ haplodont ” and “ triconodont” crowns 
are seen to-day among the Cetacea. Yet there is ground for 
uncertainty here, for as the progressive stages are “ haplodont,” 
“triconodont,” “ tritubercular,” so the retrogressive stages re- 
verse this order, passing from “ tritubercular” back to “ tri- 
conodont ” then into “haplodont.” Another view therefore is 
that such primary forms have been secondarily acquired. 
The apparently “triconodont” lower molar of Thylacinus is, 
for example, an indirect retrogression from a tritubercular 
ancestral form. Again, among the aquatic carnivora, in the 
series of molars of the Seals, the eared Seals and the Walruses, 
we see the backward stages from the “triconodont” to the 
“haplodont ;” and it is therefore probable that the “ trituber- 
cular” was the form of molar possessed by the Pinnipedia 
when they diverged from the Fissipedia. There is consider- 
able evidence that a similar retrogression has simplified the 
i a 
SSS 
Fig. 5.—Amphilestes, a Jurassic triconodont, primary. 
molar crowns of modern Edentates, for it is now certain that at 
least the Gravigrada were descended from tritubercular ances- 
tors, the Ganodonta. Again, among the Cetacea, all their oldest 
allies, such as Zeuglodon, are triconodont, not haplodont. 
With both these groups, therefore, there are therefore the possi- 
bilities of direct or of retrogressive origin of the “ triconodont ” 
molar. 
This uncertainty hardly extends to the “ triconodont” stage, 
which is typically shown in the lower Jurassic Amphilestes, 
