326 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



Injerior grinding teeth. — The most distinctive char- 

 acters of the lower grinding teeth (PI. LVI) are the 

 prominence of the paraconid, the distinctness of the 

 metaconid, and the crescentic form of the hypoconid 

 on P2 as compared with that of P. major; in more 

 progressive specimens of P. leidyi p2 is almost as pro- 

 gressive as p3 in P. major. Similarly, ps and p^ are 



gressive than those of P. major, and several stages are 

 represented in the five skulls described below. 



Some of these stages belong to animals related to 

 the type of P. leidyi; others are intermediate between 

 P. leidyi, P. major, and P. rolustus. Of para- 

 mount interest is the origin of the osseous horns. 

 First stage: Transitional skull (Am. Mus. 12185) 

 from level Bridger C 3, found at the mouth 

 of Summers Dry Creek, appears to be the 

 most primitive in its dentition, ps lacldng 

 the mesostyle, p^ comparatively primitive, 

 and the premolar-molar series measuring 

 162 millimeters, yet the sagittal crest is 

 much broader (17 mm. at narrowest part) 

 than in the P. major hypo type skull; the 

 zygomata are more massive and widely 

 expanded, and the measurements through- 

 out are more robust. In this specimen 

 the protoconules are greatly reduced and 

 the metaconules are small and lophoid. 

 It is apparently a male, the canines meas- 

 uring 36 millimeters vertically and 20 

 across the base of the crown. The molar 

 crescents are of the "wide-angle" type 

 seen in the hypotype of P. major. 



Second stage: A higher stage is repre- 

 sented by the female skull Princeton Mus. 

 10009, in which as a progressive feature a 

 very distinct mesostyle is observed on p* 

 and a rudimentary mesostyle on p^; p^-m' 

 estimated at 150 millimeters. The supe- 

 rior molars exhibit very distinct protoco- 

 nules but no metaconules. The lower jaw 

 exhibits the more oblique chin of the female 

 type, and the mandibular ramus meas- 

 ures 87 millimeters behind nis, in which 

 the hypoconulid is progressively conic in 

 form. In this specimen, however, m2 is 

 very simple. 



Third stage : The third stage exhibits the 

 horn swellings without rugosity. It is rep- 

 resented by the finely preserved, apparently 

 female skull Am. Mus. 1516 (fig. 275), in 

 which the zygomata are moderately ex- 

 panded and the sagittal crest has a mini- 

 mum breadth of 13 millimeters. More 

 in detail, the superior aspect represents 

 FiGUEE 275. — Skull of Palaeosyops leidyi an adult but not aged animal. Although 



One-fourth natural size. Am. Mus. 1516; Sage Creek, Bridger Basin, Wyo.; level probably Bridger practicaUy of the Same size in itS length (415 



mm.) it is somewhat narrower (275 mm. as 

 against 310) than the type (Am. Mus. 1544) and ap- 

 pears to be in a stage only slightly advanced beyond 

 that of skull Princeton Mus. 10009, because the 

 sagittal crest is just beginning to broaden out into the 

 plane of the vertex, the vertex of the crest measuring 

 13 millimeters transversely. The supratemporal crests 

 are very prominent, sharply overhanging the tempo- 

 ral fossae and terminating anteriorly in prominent 



C or D. Ai, Side view, reversed (canines from Am. Mus. 12185J; Aj, top view. 



much more advanced than the corresponding teeth of 

 P. major. In the true molars a marked feature is the 

 more conic form of the hypoconulid of nis. All these 

 specific characters, however, exhibit fluctuations 

 either toward a more primitive or a more progressive 

 type. 



Stages of evolution represented hy the sJculls. — All the 

 characters of the cranium of P. leidyi are more pro- 



