EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 



325 



Specific and age characters. — The materials enum- 

 erated above exhibit a considerable range of measure- 

 ment (see table on p. 316), as well as progressive 

 development of all the mutational characters. The 

 skull form in Am. Mus. 12185 is the most primi- 

 tive of the series and in many respects takes a 

 position intermediate between that of P. major and 

 that of P. leidyi; on the other hand, the type skull 

 (Am. Mus. 1544) has an extremely broad cranial roof 

 and differs from all known specimens of Palaeosyops in 

 this respect. In addition to these differences, which 

 are due to actual progressive stages of development, 

 there are apparent differences due to age. For ex- 

 ample, as explained above, the rudimentary cusplets 

 (such as the paraconid and metastylid), seen especially 

 on the unworn premolar teeth of young individuals, 

 disappear on the worn premolar teeth of old indi- 

 viduals. Another very important age character is the 

 faint frontal rugosity prophetic of a horn seen in old 

 male skulls such as Am. Mus. 1581 but absent in all 

 the younger male and female skulls. With these ex- 

 ceptions P. leidyi may be provisionally characterized 

 as follows: 



Specific cliaracters of P. leidyi hased on the type. — 

 Premolars, especially in specimens from the upper 

 levels, slightly more complex than in P. major; para- 

 conid distinct and metaconid rudimentary on p2; 

 metaconid distinct on ps; entoconid distinct on p4; 

 tritocone larger on p^ becoming convex or ridged; p^ 

 with mesostyle rudimentary or variable ; p^ with meso- 

 style distinct, sometimes large. Superior molars with 

 parastyle somewhat more prominent, ectoloph conse- 

 quently more oblique, external cingula more distinct, 

 all cingula heavier in specimens from the upper levels, 

 valleys of the external crescents somewhat narrower; 

 conules variable, often reduced, sometimes very large; 

 metaconules lophoid on m' and m^, often much re- 

 duced, sometimes very large on m'. In the skull, 

 sagittal crest variable, sometimes narrow, in the type 

 broadened into a fiat crest (figs. 276-279), occiput 

 confluent with cranial roof superiorly (in type), post- 

 tympanic and postglenoid approximated, almost 

 touching, coronoid process of jaw broadly concave 

 inferiorly. 



Incisors. — The superior series measures 70 milli- 

 meters from side to side (Am. Mus. 1544); the incisor 

 teeth increase in size from i' to i^, transversely measur- 

 ing i' 11 millimeters, i^ 12, i^ 16. Similarly the inferior 

 incisors increase slightly but regularly in height and 

 breadth, the lateral teeth being either continuous with 

 or slightly separated from the canine. There are su- 

 perior diastemata between the grinding series and the 

 canines, also between the lateral incisors and the 

 canines. 



Canines. — The canines are more slender in females, 

 measuring vertically 32 millimeters (Princeton Mus. 

 10009); the superior canines in this specimen measure 



34 millimeters. In the males the canines are more 

 robust, the fangs at the base measuring (ap. by tr.) 21 

 by 20 millimeters, and when unworn are very slightly 

 recurved, sharply pointed, differing from those of 

 Telmatherium in their circular section and strongly 

 convex inner sides. In one specimen (Am. Mus. 1549) 

 the posterior base of the crown is horizontally grooved, 

 apparently as a re- 

 sult of use of this 

 tooth in uprooting 

 plants or pidling 

 down twigs. 



Superior grind- 

 ing teeth in the type 

 and other speci- 

 mens. — P' exhibits 

 diastemata both tn 

 front and behind 

 (seePls.LX,LXII), 

 whereas in the more 

 progressive speci- 

 mens of P. rolusfus 

 the diastema be- 

 hind p^ is closed; p' 

 is occasionally 

 large (Am. Mus. 

 1552, 5102). The 

 succeeding premo- 

 lars are distin- 

 guished by sharply 

 convex protocones, 

 flattened or very 

 slightly convex 

 tritocones, internal 

 cingula variable, 

 mesostyle wanting 

 on p', variable, 

 often very distinct 

 on p^ ; external cin- 

 gula of the pre- 

 molar and molar 

 ectolophs are vari- 

 able but especially 

 strong in progres- 

 sive specimens; in figure 274.— Incisors and canines of 

 general, more Limnohyops and Palaeosyops 



sharply marked One-half natural size. A, L. priscus, Am. Mus. 

 than in P maior ^'"^^ (type); Orizzly Buttes, west Brldger Basin, 

 . '. ^ ' Wyo.; Bridger formation, lower beds. B, P. 



It IS a Strikmg fact Iddyi, Am. Mus. 1616; Sage Creek, Bridger Basin, 

 that in all the tvpi- '^' ^' '"'''''' ^™- ^^''^- IS*^ (type); Henrys Fork, 

 , . Bridger Bafin; probably Bridger upper C or D. 



cai specimens 



(Am. Mus. 1544, 1516; Princeton Mus. 10009) refer- 

 red to this species the metaconules are wanting, while 

 the protoconules are quite distinct. M^ is a large tooth 

 but still inferior in measurement and especially in de- 

 velopment of the parastyle to that of P. rohustus. The 

 series p^-m^ measures from 150 to 159 millimeters, as 

 compared with 145 to 147 in P. major and 163 to 170 

 in P. rohustus. 



