EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 



315 



Molars. — The superior molars are progressively 

 distinguished (1) by the increased development of 

 the cingula; (2) by the intensified striations of the sides 

 of the cones; (3) by the robust and rounded form 

 of the parastyle and increasing prominence of this 

 style; (4) by the relative persistence of the conic or 

 sublophoid protoconules; (5) by the more variable, 

 rounded, or lophoid metaconules^*; (6) by the subtrian- 

 gular or subselenoid form of the hypocone in m' and m^, 

 the protocone remaining rounded or bunoid; (7) by 

 the fact that in no specimen of Palaeosyops has a 

 distinct hypocone on m^ been observed, although the 

 metaconule is often enlarged and might easily be 

 mistaken for a hypocone, and the posterior cingulum 

 is often elevated. The inferior molars are readily 

 distinguished progressively (1) by the rounded, bulb- 

 ous character of the sides of the main cusps; (2) by 

 the vertical grooving or striation of the sides of the 

 cusps; (3) by the festooning of the external cingula; 

 (4) by the prominence of the paraconids, the increased 

 development of the metastylid, entostylid, and 

 parastylid; (5) by the increasingly central position 

 and rounded form of the hypoconulid in m.3, a cusp 

 which is subcrescentic when entirely unworn but, as 

 pointed out by Earle, wears into a circle in old age; 

 (6) by the median ridges strengthening the internal 

 slopes of the protoconid and hypoconid crescents, 

 which are evidently dynamically correlated with the 

 presence of the analogous median external ridges on 

 the outer slopes of the paracone and metacone of the 

 upper molars. The internal ridges also serve to 

 comminute the food by pressing against the pro- 

 tocones and hypocones. 



Slcull and jaws. — The detailed primitive and pro- 

 gressive characters of the jaw and of the male skull 

 may best be indicated under the descriptions of the 

 various species. In general, the skull (fig. 276) is 

 distinguished by (1) nasals tapering anteriorly; (2) 

 a prominent convexity above the orbits covering a 

 large frontal sinus; (3) rudimentary osseous horns 

 which appear on the sides of the face; (4) a relatively 

 short, free sagittal crest; (5) a moderately elevated 

 occiput; (6) deep and outwardly arched zygomatic 

 arches; (7) in earlier stages post-tympanic and post- 

 glenoid processes slightly separate; (8) a very large 

 mastoid foramen; (9) occipital condyles moderately 

 expanded; (10) constantly increasing expansion of 

 the masseteric insertion beneath the orbits; (11) 

 premaxUlary symphysis short and rounded. In the 

 jaw marked differences are seen between the sexes 

 (figs. 266, 267). In the males the chin is very deep 

 and prominent, correlated with the insertion of the 

 long fangs of the extremely robust canines. In the 

 females the chin is more shallow and more sloping 

 and rather resembles that of Limnohyops. The dis- 

 tinctive characters of the male jaw (fig. 268) are (1) 

 the marked curvature of the lower border; (2) the 

 great thickening of the lower border beneath the 



2* These cusps appear to be entirely wanting in the primitive species Palaeosyops 

 fontinalis, from Bridger A. 



I alveoli of the grinding teeth, which first appears 

 beneath the premolars and then extends progressively 

 backward beneath the molars; (3) the comparatively 

 short digastric fossa for the insertion of the digastric 

 muscle below the posterior portion of the symphysis; 

 (4) the greater distance between the posterior molar 

 and the back of the angle as compared with Limno- 

 hyops; (5) the breadth of the anterior border of the 



Figure 268. — Lower jaws of three species of Palaeosyops 



One-fourth natural size. A, P. longirosMs, Princeton Mus. 10275 (type), Bridger 

 Basin, Wyo.; B, P. paludosus. Am. Mus. 11680 (neotype), Millersville, Little 

 Dry Creek, Bridger Basin, level B 1; C, P. major, Am. Mus. 12181 (neotype jaw), 

 Cottonwood Creek, Bridger Basin, level B 3. 



coronoid, which becomes progressively hollowed out 

 in front; (6) the reduction of the upper portion of 

 the coronoid process, which in some higher stages 

 assumes a more simple form with an abbreviated 

 posterior hook. 



The characters of the skeleton are fully described 

 on page 619. 



QUAHTITATITE EVOLUTION OF PALAEOSYOPS 



The following table gives the measurements of 28 

 characters of proportion in 16 specimens, belonging to 

 6 species, collected at ascending geologic levels in the 

 Bridger formation: 



