EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 



333 



tion. There is a short diastema behind the canine; 

 p' is of small size. The following measurements are 

 much inferior to those of the type of P. roiustus: 

 P* 23 millimeters, transverse; width of last molar 39, 



-Fragments of jaws of 

 Palaeosyops 



One-fourth natural size. A, P. granger!, Am. Mus' 

 12189 (type); Twin Buttes, Bridger Basin 

 Bridger formation, level CI. B, P. roSus/MS, Am- 

 Mus. 1590; Bridger Basin, Wyo.; inner view. 



as in P. leidyi; small conules on the molars; a faint 

 "shadow" rudiment of the mesostyle on p^. 



Second mutation: Am. Mus. 1558 also exhibits a 

 faint "shadow" mesostyle rudiment on p^; a very 

 strong mesostyle on p*. The following measurements 

 are similar to those of P. leidyi: Width of p* 26 mil- 

 limeters; width of m' 39. Small conules on the molars. 

 P' is unusually large. 



Third mutation: Am. Mus. 1552 is the palate of a 

 small female individual. Premolars and molars ex- 

 hibit strong external cingula; mesostyle is rudimentary 

 but distinct on p^, very strong on, p*. The measure- 

 ments are, however, the same as in P. leidyi, namely, 

 p*, transverse, 23 millimeters; m^ transverse, 39. 



Fourth mutation: Am. Mus. 1584 exhibits an 

 advanced mesostyle on p^ and p''. The diastemata 

 are closed up, as in the type of P. roiustus, and p* 

 shows a slightly increased width (27 mm.) transversely. 

 The molars exhibit very small conules and faint 

 cingula. 



Detailed characters of the type and other progressive 

 forms. — The dentition of the type specimen is fully 

 described above and figured on Plates LVI, LVIII. 

 In this specimen p* attains a width of 26 millimeters, 

 and m^ a width of 41; the measurement of m^ as 

 seen in its oblique diameter, measured from the para- 

 style to the hypocone, is 52 as compared with 48 

 in P. leidyi and 56 in the type of P. grangeri. This 

 diagonal expansion of m^ is characteristic of the spe- 

 cies. The mesostyles of p^ and p* are worn off or 

 possibly were not present in the type. The cingulum 



nearly closes in around the inner sides of the premolars. 

 Both pro to- and metaconules on the molars are large. 



Stages similar to the type : Very close to the stage 

 represented by the type is the dental series Am. Mus. 

 11683 (level Bridger D 3), measuring, p^-m^ 167 

 millimeters; width of m', 42; width of p"*, 27. 

 The cingula are progressive on the inner side of p*; 

 as in the type the postcanine diastema is very narrow, 

 and, as observed above, the parastyle expansion is very 

 marked. The masseteric ridge of the malar is very 

 deep below the orbit. 



In a similarly advanced stage is Princeton Mus. 

 10282b, with heavy cingula and a large mesostyle 

 on p*. 



Cope's type of Palaeosyops diaconus. Am. Mus. 

 5106 (Henrys Fork, Bridger Basin, Wyo., level 

 Bridger D?), is also very progressive, with "shadow" 

 mesostyle on p' (nearly worn off) and p^. The 

 internal premolar cingula are nearly in contact on the 

 inner sides of the deuterocones; similarly the cingula 

 nearly embrace the protocones of the molars internally. 

 The transverse measurement of p* is 26 millimeters. 



FiGtiRE 285. — Skull of Palaeosyops rohustas 

 Top view. One-fourthnatural size. Am. Mus. 1S64; Henrys Fork, 

 Bridger Basin, Wyo.; Bridger formation, probably level D. A-A, 

 Section tine across born swelling (cf . fig. 210) . 



A pecuhar feature is the expanded metaconule of m^ 

 which is unfortunately broken away in the posterior 

 half. Cope believed that this metaconule represented 

 a second internal cusp or hypocone, as in Limnohyops; 



