496 The American Naturalist. [June, 
triconodont type, are marked step by step by the division of 
the fang. This law was advanced hypothetically by Cope and 
Wortman, and I regard it as absolutely proven by the evi- 
dence I have adduced in the study of Dromotherium, Microcon- 
odon and Amphilestes. 
The hypothesis of Kiikenthal and Rose that the numerous 
single pointed or homodont teeth of the whales, have arisen 
by the splitting up of the three cusps of a triconodont crown 
is an ingenious one, upon which paleontology at present 
throws no favorable light. Amphilestes of the middle Triassic 
with its seven triconodont molars, might by such a splitting 
process, furnish twenty-one homodont teeth; nevertheless, 
this seems to me highly improbable; while the converse 
hypothesis suggested by Kiikenthal and developed by Rose, 
that multicuspid crowns have originated by the fusion of 
single cusps, is capable of direct disproof by paleontological 
evidence. 
The multiplication of teeth accompanying the elongation of 
the jaw in Cetacea, can be much more simply explained by 
the supposition that the dental fold was carried backward, 
and gave rise to new teeth caps at definite intervals. I may 
add that the rapid reduction of the molars in the Mesozoic 
period from behind forward, which reduced their number in 
the Triconodontide from seven to four, between the middle 
and upper Jurassic periods, is against the supposition that 
the Amphilestes molar, for example, furnished the material for 
the multiple Cetacean series. 
Kikenthal’s researches upon the dentition of the opossum, 
published in 1891, mark another great step in advance in 
our knowledge of the dentition of the mammals. We may 
refer to the later researches of Röse for details, and simply 
quote one passage from Kiikenthal: 
“The permanent dentition of the Marsupials, belongs te the 
first series or milk dentition. Rudiments of the second den- 
tition are actually present in an embryonic condition, but 
with the exception of the third premolar, they do not cut 
the gum.” . Again, the two first so-called true molars, of the 
*Anatomischer Anzeiger, Nos. 23, 24. 
