EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERE3 



271 



The primitive superior premolars of Eotitanops 

 (fig. 229, A) when contrasted with those of Bron- 

 totherium (fig. 229, B) are seen to be triangular in 

 outline and surmounted with three more or less 

 perfectly developed cusps. The transition between 

 these primitive and specialized extremes, by the addi- 

 tion of neomorphs or rectigradations, furnishes a 

 whole series of specific characters and mutations in 

 the ascending phyla of titanotheres. 



All the phyla exhibit a similar orthogenic tendency to 

 molarization, but in some phyla this tendency is rapid 

 and in others it is slow. Thus the different rates of 

 molarization are of taxonomic value: they furnish 

 distinctions between different generic or phyletic series. 

 (See Velocity of movement. Chap. XI, pp. 810-812.) 



A careful review of the premolars in lower and 

 middle Eocene titanotheres shows that the order of 

 appearance of the premolar cusps is as follows: 



Lower premolars Upper premolars 



L Protoconid. 1. Protocone. 



2. Hypoconid. 2. Deuterocone. 



3. Paraoonid. 3. Tritocone. 



4. Metacristid. 4. Parastyle. 



5. Metaoonid. j 5. Tetartocone. 



6. Entooonid. I 6. Mesostyle. 



In the above terminology of the upper premolar 

 cusps the names are those proposed by Scott in 1892 



in his paper on the evolution of the premolar teeth 

 in the mammals (1892.1), but they are used through- 

 out this monograph simply as names, without re- 

 ference to the phylogenetic order of evolution, which 



Figure 229. — Upper premolar-molar teeth of the earliest (A) 

 and latest (B) known titanotheres 



Both tooth rows drawn to the same length. A, Eotitanops lorealis, Am. Mus. 

 14887; lower Eocene. Premolars very primitive, a wide diastema behind pm'. 

 B, Brontotlicrium gigas. Am. Mus. 492; lower Oligocene. The later type, showing 

 the molarized pattern of the premolars and the great size of the molars. Pre- 

 molars very advanced in pattern, diastema closed. 



is given above. The names of the lower premolar 

 cusps are adapted from the terminology used by' 

 Osborn for the lower true molars, but again without 

 reference to the phylogenetic order of appearance. 



