EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHBRBS 



425 



Measurements of species of MesatirJiinus and MetarTiinus, in millimeters 



[Specimens in Am. Mus. except 12195, which is in the Field Museum] 



Incisive border to angle 



Incisive border to condyle 



Posterior border of jaw to ms- 



Depth below ma 



Pi-m3 



Mi-m3 



Mesatirhinus 



M. mega- 

 rhinus, 

 Bridger 

 No. 1520 



162 

 94 



M. petersoni, Bridger 



No. 1567 No. 1512 



132 



74 

 168 



325 



112 



83 



172 



103 



325 

 325 

 -90 

 79 

 169 

 106 



M. sp., 

 Uinta B, 

 No. 1859 



"SSO 



350 



90 



86 



195 



MIS 



M. fluviatilis, Uinta B 



161 

 102 



M. riparius 

 Uinta B 1 



(upper 



Metarhinus 



beds), 



No. 12195 



280 



285 



78 



62 



157 



338 



172 

 110 



These jaws are distinguished by the laterally con- 

 stricted chin, correlated with the constricted premax- 

 illaries above; also by the slender rami with the lower 

 borders slightly downcurved below the grinders and 

 upcurved below the coronoid, extending back into an 

 angle which is produced posteriorly, as in Mesati- 

 rhinus. With the exception of the chin, the jaw, so 

 far as known, was of the Mesatirhinus type. The an- 

 terior border of the coronoid is decidedly angulate in 

 character; the coronoid itself rises with nearly parallel 

 anterior and posterior borders to a gently recurved 

 tip. These characters suggest those of a dwarfed col- 

 lateral of M. megarhinus. In the supposed female 

 (Am. Mus. 2059) the ramus appears much deeper 

 and more slender than in Am. Mus. 1946. 



The type of Heterotitanops parvus Peterson — is it a 

 young Metarhinus? — A very young, perhaps a fetal 

 skeleton in the Carnegie Museum (No. 2909), the 

 type of Heterotitanops parvus Peterson (figs. 150-152), 

 may possibly represent Metarhinus fluviatilis. It was 

 found lower down in Uinta B 1 than any other known 

 mammalian remains. The type of Metarhinus flu- 

 viatilis was also found by Mr. Peterson in Uinta B 1. 

 According to W. K. Gregory its reference to Meta- 

 rhinus or to some closely allied genus is indicated by 

 the following facts : 



1. The deciduous cheek teeth, in size and appear- 

 ance, bear to the adult dentition of Metarhinus fluvi- 

 atilis much the same relation as the deciduous denti- 

 tion of Oligocene titanotheres (PI. XXIII) bears to 

 the permanent dentition — that is, the deciduous molars 

 are more molariform than the permanent premolars, 

 and they are elongate anteriorly and have more widely 

 open external V's and less prominent mesostyles. 



2. The large upper and lower grinding teeth that 

 have not wholly emerged from the alveoli are prob- 

 ably not m' and mi, as supposed by Peterson, but dp* 

 and dp4. The anteroposterior diameters as given by 

 Peterson are dp* 21 millimeters, dp4 25, dimensions 

 nearly as great as in m' and mi of the Metarhinus fluvi- 



atilis type and apparently large enough for dp* and dp4 

 of that species. The resemblance of dp*, dp4 to m' and 

 mi of that species seems sufficiently striking to indicate 

 congeneric relationship. The measurements of dp^ 



Figure 358. — Lower jaws of Metarhinus 



One-fourth natural size. A, M. fiumatilis. Am. Mus. 2059, White River, 

 Uinta Basin, Utah, Uinta B 1; B, M. earlei. Am. Mus. 13179, northwest 

 point of Haystack Mountain, Washakie Basin, Wyo., Washakie B. 



are also not inferior to those of dp4 in Mesatirhinus sp. 

 (Am. Mus. 12211). 



3. The deep lateral excavation of the anterior 

 nares, which leaves a very narrow bridge of bone be- 

 tween the narial sinus and the orbit, a feature char- 

 acteristic of Metarhinus and its near aUies, is strongly 

 marked also in the specimen under consideration. 



