EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 



395 



The condyle in M. petersoni is well raised (144 mm.) 

 above the lower border of the angle and extends 107 

 millimeters behind ms; it is more extended antero- 

 posteriorly and less transversely than in M. manteo- 

 ceras. The angle is very similar to that of Eotitanops 

 borealis on a larger scale, with a slender and slightly 

 incm-ved posterior border. The lower border of the 

 ramus is concave below the coronoid, convex below 

 the grinders, and rises gradually into a laterally com- 

 pressed chin gently rounded on the inferior surface. 

 The ramus increases in depth posteriorly. The 

 symphysis measures 69 to 80 millimeters, and as seen 

 from below the chin is sharply contracted to a width of 

 40 millimeters behind the canines. Below mi the 

 rami attain in males the width of 20 millimeters. On 

 the whole this is a progressive development of the E. 

 borealis type of jaw, the chief difference being the 

 broader coronoid. 



Age characters. — In an aged, somewhat larger, more 

 elongate, and perhaps more progressive jaw (Am. 

 Mus. 1567), from the Bridger, there is a wider space 

 (130 mm.) between the condyle and the posterior 

 fang of ms, the condyle itself is wider (57 

 mm.) and less extended anteroposteriorly, 

 the gentle rounding of the posterior border 

 of the angle seen in E. borealis and the 

 typical M. megarJiinus changes into a more 

 decided, angulate projection of the postero- 

 inferior border. Seen from behind, the 

 border of the angle is marked by a sudden 

 sharp inflection about two-thirds of the 

 distance below the condyle 



occipital condyles measure 98 millimeters transversely, 

 as compared with 86 in M. petersoni. The width 

 across the postglenoid processes is 183 millimeters, as 

 compared with 150 in M. petersoni. 



This cranium may possibly belong to an animal 

 with a dentition such as that which we have referred 

 below to D. vallidens. 



A progressive jaw from Washalcie A. — A specimen 

 (Am. Mus. 2355) from Washakie A at Glove Springs 

 consists of the rami incomplete posteriorly, but includ- 

 ing all the teeth. It belongs to the dolichocephalic 

 Mesatirhinus series. It is much larger than the most 

 advanced jaw of M. petersoni from Washakie A. The 

 measurements are compared below: 



Figure 332. — Lower jaw of Mesatirhinus sp. with deciduous dentition 

 The COndvle One-half natural size. Am. Mus. 12211, Summers Dry Creek, Bridger Basin, Wyo., Bridger C. 



is more transversely extended. 



A small lower jaw (Am. Mus. 12211), from Bridger 

 C, has deciduous incisors 1 and 2 and three deciduous 

 premolars in place, with some of the replacing teeth 

 below them. The incisors are chisel-shaped, some- 

 what like the adult incisors of Lambdotherium. The 

 fourth deciduous premolar is more molariform than 

 its successor, especially in the somewhat better devel- 

 opment of the entoconid. The chin is very sloping. 

 (See fig. 332.) 



Mesatirhinus sp. 



Large progressive sTcuil (fig. 333). — There is interest- 

 ing evidence (Princeton Mus. 10041) of the existence 

 in Washakie B (?) of a much larger animal than M. 

 petersoni, progressive at least in size toward Doliclio- 

 rJihius vallidens. It differs from MesatirTiinus superior 

 in the narrow sagittal crest. 



Unfortunately only the occiput is preserved. It ex- 

 hibits in the parietal profile a pronounced convexity; 

 the parietal crests are also broadly divergent anteriorly 

 and rounded, suggesting those of SpTienocoelus. 



The superior dimensions are indicated by the follow- 

 ing comparisons: The occiput measures 90 millimeters 

 in height, as compared with 80 in M. petersoni; the 



Measurements of Mesatirhinus and Dolichorhinus, in millimeters 



Pi-ms- 

 Pz-ma- 



Mi-ms. 



175 

 164 

 103 



192 



177 

 112 



Washakie B: 

 D. hyogna- 



thus, Prince- 

 ton Mus. 

 10273 (type) 



240 

 213 

 120 



The postcanine diastema in this specimen is -long 

 (32 mm.). As in MesatirTiinus the canines are incurved 

 as well as recurved. 



Mesatirhinus ( = Dolichorhinus) superior Riggs 



Reference may be made here to the skull of M. 

 { = Dolichor7iinus) superior, which is fully described 

 below (p. 405). This animal is intermediate in form 

 between the two genera Mesatirhinus and Dolicho- 

 rhinus, so that it might be placed in either genus with 

 equal propriety. The skull and hypocone on m' of 

 M. superior incline us to connect this skull with species 

 of Dolichorhinus. 



