EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 



393 



ridges, a feature which at once distinguishes them from 

 the lower canines of T. cultridens, in which these ridges 

 are prominently marked. 



Premolars. — The superior premolars especially ex- 

 hibit the progressive rectigradations in the new cus- 

 pules, which, as well as the progressive changes of 

 proportion, are subject to slight fluctuations. In the 

 more advanced American Museum specimens {M. 

 petersoni) the first superior premolar is separated from 

 the canine by a narrow diastema, whereas in the less 

 advanced Princeton Museum type of M. megarJiinus, 

 which represents a less dolichocephalic stage, it is 

 in actual contact with the canine. The detailed 

 characters are as follows: P' is bifanged, with a 

 simple protocone, strongly compressed laterally, in 

 which the proportions are typically 14 millimeters 

 anteroposterior by 9 transverse; in the more pro- 

 gressive specimens (M. petersoni) the tritocone (a 

 rectigradation) is seen as a rudimentary swelling of 

 the posterior base of the crown, which is less con- 

 spicuous in M. megarJiinus. P^ is a highly character- 

 istic tooth, suboval or slightly compressed trans- 

 versely, the proportions (ap. by tr.) varying in different 

 specimens from 15 by 13 to 16 by 16 millimeters; 

 the proportions of this tooth are those correlated 

 with mesaticephaly progressing into dolichocephaly; 

 it is typically tricuspidate (protocone, deuterocone, 

 tritocone); a generic feature is the excess of the large 

 conic protocone over the small, externally flattened 

 tritocone; the ectoloph is slightly cingulate (M. 

 petersoni) but lacks the strongly accented cingulum 

 around the base of the tritocone seen in T. cultridens 

 and T. validum. P^ is naturally a more progressive 

 tooth, the breadth exceeding the length (ap. 14 

 millimeters, tr. 18, M. megarhinus ; ap. 17, tr. 20, 

 M. petersoni), the tritocone and protocone com- 

 ponents of the ectoloph being more subequal, the 

 basal external cingulum opposite the tritocone being 

 more accented, and the deuterocone being more directly 

 internal in position. P* still further marks this 

 progression toward the molar type in its dimensions — 

 17 by 22 millimeters (ap. by tr.) in M. megarhinus, as 

 compared with 18 by 24 in M. petersoni. The external 

 cingulum, varying in both species, is either partially 

 indicated (Am. Mus. 1523, 1571) or extends across 

 the outer face of the crown (Am. Mus. 1514, 1556); the 

 less progressive individuals (Am. Mus. 1523, 1513, M. 

 megarJiinus) pass into more progressive stages (Am. 

 Mus. 1556, 1509, M. petersoni) in which a faint rudi- 

 ment of the protoconule is observed in p^, p^, and in 

 Am. Mus. 1556 {M. petersoni) even a faint elevation 

 of the tetartocone is observed (a rectigradation). 

 Similarly the convex external rib of the protocone 

 becomes more marked. 



In comparing the premolar series in all these 

 specimens it is seen that the external cingulum 

 exceptionally almost or quite embraces the ectoloph, 



but that the internal cingulum never completely 

 embraces the deuterocone, as in the type of MetarJiinus 

 fluviatilis. The premolar cingula are on the whole as 

 progressive or more progressive than those of T. 

 cultridens and D. vallidens. Another important pro- 

 gression is seen in the premolar ectolophs — namely, 

 in certain specimens, Am. Mus. 1556, 1509 {M. 

 petersoni) the tritocone ectoloph is flat, as in D. 

 vallidens (Cope), whereas in other specimens, Am. 

 Mus. 1571 (if. petersoni), 1513 {M. megarJiinus), 

 12184 (type of M. petersoni), the tritocone ectoloph is 

 more conic, as in M. manteoceras. 



Comparative measurements of the superior pre- 

 molars are given in the table on page 388. 



The inferior premolars are more or less perfectly 

 represented in six jaws in the American Museum 

 collection, none of which, however, are certainly 

 associated with skulls. Pi is a small, conic or slightly 

 flattened tooth, separated from the canine by a diastema 

 8 to 12 millimeters in length; a slight diastema (4 

 mm.) also separates it from P2; pi is a typically single, 

 rarely bifanged tooth, with a narrow, laterally com- 

 pressed, recurved, pointed crown (9 by 6 mm.). P2 is 

 a bilobed tooth and elongate, but relatively less so 

 than in T. cultridens — in fact, it is slightly more pro- 

 gressive than in that species; the typical measure- 

 ments are 18 millimeters anteroposterior and 9 

 transverse; anterior to the elevated protocone is the 

 beginning of the anterior valley and a rudiment of 

 the antero-internal cusp ( = paraconid) ; the much more 

 depressed talonid similarly consists of a shallow, 

 rudimentary crescent, opening inward. P3, like its 

 fellow in the upper jaw, shows more equal anterior 

 and posterior lobes, on which the crescents and 

 internal valleys and the cusps corresponding to the 

 paraconid and entoconid of the molars are more 

 accented; the typical proportions in M. megarJiinus 

 are 17 by 9 millimeters; in M. petersoni the typical 

 proportions of p2 are 19 by 10, but this tooth has 

 only a rudiment of the prominent internal cusp 

 corresponding with the metaconid of the molars. 

 P4 is still further advanced or submolariform, having a 

 prominent internal cusp corresponding to the meta- 

 conid in the molars, each lobe consisting of two fairly 

 defined crescents ; it differs from mi in its smaller dimen- 

 sion {M. megarJiinus 17 by 11 mm., M. petersoni 19 by 

 12) and in the nonelevation of the postero-internal cusp 

 (entoconid). The cingulum is practically rudimentary 

 or wanting in all these teeth. 



Molars. — The MesatirJiinus or generic characters of 

 the superior molars (PI. LXXII) are seen in the fol- 

 lowing features: (1) The slight excess of anteropos- 

 terior over transverse diameters, especially in the more 

 dolichocephalic M. petersoni; (2) the high, sharply 

 pointed protocone (unworn height, 6 mm.); (3) the 

 high, elongate external cusps (height of unworn para- 

 cone, 23 mm.); (4) the very sharp para-, meso-, and 



