TRITUBERCULAR EVOLUTION IX MAMMALS 37 



retention of it in the E<l<'nl,it, 1 * support all the independent evidence 

 upon this point derived from the Mesozoic Mammals. 



The principle of growth was the regular addition of new parts to 

 the simple cone, not at random, but according to a certain definite 

 order which apparently progressed independently in different phyla, 

 through a series of subtritnbercular stages until trituberculy - was 

 attained. 



The tritubercular molar consists essentially of three cusps, forming 

 what may be called the primitive triangle, 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 Ceta- 

 ceans, although there is considerable evidence that the cetacean 

 molars were once of the triconodont type. 3 t Among extinct orders, 

 the Multituberculata (P/i./r/in/'/n.,', Triti/loclon, 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, 4 all except three 5 J show trituber- 

 culy in some of its stages. In the Lower Eocene, eighty-two Puerco 

 species, representing twenty-six genera and five orders (Creodonta, Tillo- 

 dontia, Lemuroidea, Condylarthra, Amblypoda), only four species have 

 quadri tubercular teeth, all the remainder are tritubercular. 6 Prof. 

 Kiitimeyer has recently pointed out the predominance of this type 

 in the nearly parallel Egerkingen beds. The contemporary Cernaysien 

 fauna in the collection of Dr. Lemoine at Ptheims, recently examined 

 by the writer, shows exclusively tritubercular molars or their deriva- 

 tives. By the Middle Eocene, the lines of divergence towards the 



: See Oldfield Thomas, "The Homologies and Succession of the Teeth in the Dasyu- 

 ridre," Phil. Trans., 1887, p. 458. 



* [Evidence is presented on pp. 151, 190, that the haplodonty of certain Edentata and 

 of the Cetacea is secondary. ED.] 



2 A term first employed by Riitimeyer, " Ueber Einige Beziehungen zwischen den 

 Saugethierstammen Alter und Neuer Welt," Abh. d. schweiz. pat. Gesellsch., Vol. XV., 

 1888, p. 54. 



3 See Brandt, Die Fossilen u. Subfoss. Cetaceen Europas, Taf. XXXII., Figs. 4-9. 



t[But this in Zeur/lodon was probably a secondary derivation from the tritubercular 

 type. See p. 191. ED.] 



4 The list given by the writer (op. cit., p. 247) is found to contain several synonyms. 

 See "Additional observations upon the Structure and Classification of the Mesozoic 

 Mammalia," Proc. Phila. Acad., Nov., 1888, p. 292. 



5 Dicrocynodon (Diplocynodon), Docodon, Enneodon, Marsh. 



I [The author refers only to the orders Protodonta, Triconodonta, Pantotheria. ED.] 



''Cope, "Synopsis of the Vertebrate Fauna of the Puerco Series," Am. Phil. 

 Soc., 1888, p. 298. 



