EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 



329 



incisors exhibit a compact transverse series measuring 

 53 millimeters. In p, the paraconid is well marked. 

 The worn grinding teeth, although perfectly preserved, 

 in such a specimen as the type do not present the 

 distinctive characters of the series; but they demon- 

 strate most conclusively that every element in the 

 crown finally comes into some degree of use in the 

 comminution of food and therefore has an adaptive 

 significance; the styles, the cingula, the cones, the 

 crescents, the conules, all are distinctly worn and 

 blunted either by opposing elements in the lower 

 teeth or by attrition of the food. The crown in this 

 stage of wear presents an ineffective grinding and 

 cutting apparatus and serves little more than a 

 crushing function, because the low crenulated cres- 

 cents and cones exhibit none of the hypsodont tendency 

 so characteristic of the Telmatherium series. 



Fifth stage: Mutation transitional to P. robustus: 

 The male skull. Am. Mus. 1581 (represented in fig. 

 281), exhibits as its most interesting and important 

 feature a rugosity and a very slight elevation of the 

 frontals just behind their junction with the nasals, 

 which represents the horn rudiment in this species in 

 the incipient rugose stage. Viewed from above (fig. 28 1 , 

 A2) this rugosity is seen to be very slightly convex — 

 that is, it rises above the surrounding surface as an 

 elevation involving the posterior border of the nasals 

 and a portion of the nasofrontal suture. This horn 

 is thus slightly posterior to the position which it occu- 

 pies in the skull of the allied genus Telmatherium. In 

 this skull the facial convexity is very prominent. 

 Although m3 is well worn, the conules are persistent 

 and very slightly affected by the wear. The meso- 

 style on p*, originally present, has been worn away. 



Comparison with dentition of other forms: Tho 

 measurements of several other maxillary series are 

 given in the table on page 316. Among the large num- 

 ber of specimens examined some (such as Princeton 

 Mus. 10009) are smaller and are more primitive in 

 structure, approaching P. major, whereas others, such 

 as Am. Mus. 5102 (P. leidyi advanced, but no meso- 

 style on p*) are larger and more progressive. In the 

 former the ectoloph and styles of the premolars are 

 less strongly developed; in the latter the ectoloph, 

 styles, and cingula are very strongly developed. In 

 Am. Mus. 12208, from C 4, the protoconules are pres- 

 ent in the second and third molar teeth. In Am. Mus. 

 5102, an aberrant form, the third and fourth superior 

 premolars are without mesostyles but at the same 

 time exhibit a very marked tendency toward molari- 

 zation and have strongly pinched ridges opposite 

 the protocones and tritocones. This specimen is pro- 

 gressive in the great prominence of the parastyle. 



Fluctuations and progressive characters in the in- 

 ferior teeth: In the several fine jaws referred to this 

 species (Am. Mus. 1585, 1564, 1546, 1549) we observe 

 fluctuations or individual variations of mutative and 



specific characters. In P2 the paraconid is always dis- 

 tinctly marked; in ps the protolophid and hypolophid 

 crests are very well defined; in p4 the metaconid ap- 

 pears in a rudimentary cuspule; in ms the hypocon- 

 ulid varies from a rounded (progressive) to a more 

 crescentric (primitive) form; No. 1549 exhibits the 

 triangular form similar to that seen in the type jaw; 

 in No. 1585 we observe the more recurved form seen in 

 P. major. 



Figure 279. — Skulls of Palaeosyops major and P. leidyi 



One-fourth natural siie. A, P. major, Am. Mus. 12182 (neotype skull); 

 middle Cottonwood Creek, Bridger Basin, Wyo.; Bridger formation, level 

 B 3; occipital view, crushed downward. Bi, P. leidyi, Am. Mus. 1544 

 (type); Henrys Fork, Bridger Basin; Bridger, level probably upper C or 

 D; occipital view. B2, The same, front view. 



Juvenile dentition and crania: In many ungulates 

 the deciduous premolars are more molariform than 

 their permanent successors, and this law is well 

 illustrated in Palaeosyops, as in the milk dentition 

 referred to P. leidyi (see below) dp^, dp', and especially 

 dp"* are more molariform than their successors p^, p', p*. 

 It seems not impossible that p^ in the titanotheres 

 represents a persistent milk tooth, or dp^ In the 

 milk dentition under discussion (Am. Mus. 1565) the 



