EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 



323 



embracing inner sides of superior premolars, tritocone 

 on p^ small, no external ciagulum ; ectolophs of molars 

 with "wide-angle" or open crescents; parastyle but- 

 tresses not very prominent; rudimentary external 

 cingula opposite valleys; the protoconules very large, 

 angulate; the metaconules small. 



The skull is that of a young adult; in an old adult 

 the masseteric ridge below the zygoma would be more 

 strongly developed. It exhibits clearly the convexity 

 of the forehead, the abbreviated sagittal crest, the 

 broad, low occiput, the separation of postglenoid and 

 post-tympanic processes, the absence of even a rudi- 

 ment of the froiitonasal horn; and these characters in 

 connection with its inferior size indicate a decidedly 

 lower stage of development than that of the skull of 

 P. leidyi or P. rohustus. (See table on p. 316.) 



The superior cutting teeth are unknown. 



Superior premolar-molar series. — This series is well 

 represented in the neotype skull (Am. Mus. 12182), 

 possibly also in Am. Mus. 5105, Cope's paratype 

 of P. laevidens, and in Princeton Mus. 10276. The 

 molars are readily distinguished from those of P. 

 leidyi and P. rohustus by their smaller dimensions. 

 (See table, p. 316.) As pointed out above, the pre- 

 molars are distinguished by the absence of well 

 developed mesostyles in p^~^ and by the smaller size 

 of the tritocone of p". 



Measurements of another specimen. — A bea-utifully 

 preserved superior premolar-molar series (Am. Mus. 

 2361, Pis. LVIII, LXII), probably from level A of the 

 Washakie Basin, is in a more advanced stage of pre- 

 molar evolution than P. major and differs from the 

 neotype of that species in having a larger tritocone 

 on p^, a larger deuterocone and stronger "protoconule 

 ridges" onp^~*, more prominent protoconule ridges on 

 m'~^ and a well-marked metaconule ridge on m'. The 

 specimen also differs in details from those referred to 

 P. leidyi, P. rohustus, and P. copei, and it may repre- 

 sent a new species or subspecies characteristic of 

 Washakie A. Comparative measurements of this 

 specimen are as follows: 



Comparative measurements of species of Palaeosyops, in milli- 

 meters 



Palaeosyops leidyi Osborn 



Plates XVI, XXVII, XLIV, L, LIII, LVI, LIX-LXII; 



te.xt figures 27, 28, 33, 118, 217-220, 227, 254-256, 273-283, 

 305, 482, 483, 485, 511-513, 520, 522, 523, 536, 537, 539, 

 540, 643, 546, 552, 559, 645, 661, 703, 713, 716, 727, 737, 

 741, 742, 745 



(For original description and type references see p. 181. For skeletal characters 

 see p. 620] 



Type locality and geologic horizon. — Henrys Fork, 

 Bridger Basin, Wyo.; Bridger formation, Uinta- 

 therium- Manteoceras- MesatirTiinus zone. Bridger levels 

 C 2, C 3, C 4, and C 5?, as exposed on Henrys Fork in 

 the Bridger Basin, are the geologic levels of this species, 

 which is well above that of Palaeosyops major — ■ 

 approximately 200 feet. 



Specific characters. — Of larger size; total length of 

 skull 415 millimeters; p^-m', 158; P2-m3, 168. Diaste- 

 mata behind canines. Posterior superior premolars 

 with mesostyles. Barely defined swellings repre- 

 senting the rudiments of osseous frontonasal horns. 

 At least four new numerical characters, or rectigrada- 

 tions, in the grinding teeth. Cephalic index, 74. 



This species is named in honor of Joseph Leidy, the 

 founder of American vertebrate paleontology and 

 first contributor to our knowledge of the titanotheres. 

 P. leidyi is noteworthy as the earliest form to have the 

 visible beginnings of horns (PL XVI, fig. 281). It is 

 the only species of which the bony structure is known 

 in nearly every part; this knowledge is obtained 

 chiefly from materials collected by the American 

 Museum expeditions under Dr. J. L. Wortman. 



The form of the occiput in the type skull (figs. 277, 

 279, Bi) is very exceptional and may be an individual 

 variation. The premolars of P. leidyi are more 

 advanced than those of P. major, but this and the 

 incipient horn bosses are about the only decisive 

 characters separating the two. The average size of the 

 skull in P. leidyi was larger than in P. major, but 

 exceptionally large individuals of P. major are actually 

 larger than small individuals of P. leidyi (see table on 

 p. 316); in fact, P. major and P. leidyi might by some 

 systematists erroneously be regarded as successive 

 mutations (in the sense used by Waagen) from 

 P. paludosus rather than as species in the Linnaean 

 sense, and this conception might be conveniently 

 expressed by trinominal names, such as P. paludosus 

 paludosus, P. paludosus major, and P. paludosus 

 leidyi. 



In the opinion of the present author we should hold 

 a heredity conception — that of germ evolution through 

 the independent advance of a very considerable num- 

 ber of single characters, including new rectigradations, 

 such as cusplets on the teeth and horn bosses on the 

 skull; and new proportions or quantitative characters 

 (allometrons). Expressed in another way, P. leidyi 

 succeeded P. paludosus after a vast interval of time, 

 as indicated by the intervening 400 to 600 feet of 

 sediment. In the long .series of generations that 

 separated these stages new tendencies of character 



