1068 Evolution of Mammalian Molars. 
order which apparently progressed independently in different phyla, 
through a series of sub-tritubercular stages until trituberculy ' was 
attained. 
The tritubercular molar consists essentially of three cusps, form- 
ing what may be called the primitive triangles, so disposed that 
the upper and lower molars alternate. This, when attained, formed 
a central stage from which the great majority of recent molar types 
have diverged by the addition, modification and reduction of cusps; 
we must except the Monotremes, the Edentates, and possibly the 
Cetaceans, although there is considerable evidence that the cetacean 
molars were once of the triconodont type.? Among extinct orders, 
the Multituberculata (Plagiaulax, Tritylodon, etc.) must also be 
excepted from this series and discussion. 
The almost universal predominance of trituberculy in the early 
geological periods, is very significant of the uniformity of molar 
origin. Of twenty known Mesozoic genera,’ all except three * show 
trituberculy in some of its stages. As to the Lower Eocene, eighty- 
two Puerco species, representing twenty-six genera and five orders 
(Creodonta, Tillodontia, Lemuroidea, Condylarthra, Amblypoda), 
only four species have quadritubercular teeth, all the remainder are 
tritubercular.’ Prof. Riitimeyer has recently pointed out the pre- 
dominance of this type in the nearly parallel Egerkingen beds 
The contemporary Cernaysien fauna in the collection of Dr. Le- 
moine at Rheims, recently examined by the writer, shows exclu- 
sively tritubercular molars or their derivatives. By the Middle 
Eocene the lines of divergence towards the existing types of molars 
were well advanced, but trituberculy persisted in the dentition of 
several orders, in which it is found to-day (Lemuroidea, Insectivora, 
Carnivora, and many Marsupialia). | 
1 First employed by Riitimeyer, “Ueber Einige Bezi ehungen 
zwischen den Säugethierstämmen Alter und Neuer Welt.” Abh. d. 
schweiz. pal. gesellsch., Vol. XV., 1888,"p. 54. 
2 See Brandt, “Die Fossilen u. Subfoss. Cetacean Europas.” Taf. 
XXXIL,, figs. 4-9. 
3 The list given by the writer (op. cit., p. 247) is found to contain sev- 
eral synonyms. See ‘ Additional observations upon the Structure an® 
Classification of the Mesozoic Mammalia.” Proc, Phila. Acad., Nove 
1888, p. 292 
Dicrocynodon (Diplocynodon), Doeodon, Enneodon, Marsh. > 
5 Cope, ‘‘ Synopsis of the Vertebrate Fauna of the Puerco Series, 
Am. Phil. Soc., 1888. p. 298. 
