EVOLUTION or THE SKULL AND TEETH OF EOCENE TITANOTHERES 



357 



m^, m', and by the detailed form of m^ From those 

 of the contemporary Manteoceras uintensis they are 

 distinguished by their smaller size, by the greater rela- 

 tive breadth of m^, more quadrate contour, especially 

 of m^ relatively smaller parastyles and mesostyles, 

 sharper external cingula. M^ is wide anteriorly (53 

 mm.) and narrow posteriorly; the posterior V is rela- 

 tively small. The molars of MetarMnus earlei are 

 relatively longer anteroposteriorly. 



Comparative measurements of Telmatherium and Sthenodectcs, in 

 millimeters 



■ Basal length of skull 



Zygomatic breadth of skull. . 



Cephalic index 



Length of dental series (i'-m^) . 



P'-m3 



P'-p* 



M'-m3 



I', ap. by tr 



P, ap. by tr 



P, ap. by tr 



C, ap. by tr 



C, vertical 



P*, ap. by tr 



Ml, ap. by tr 



M^, ap. by tr 



M^, ap. by tr 



T. ultimum, 

 Am. Mus. 

 2060 (type) 



500 



300 + 



60 



305 



218 



89 



130 



14X13 



15X14 



19X17 



* 25X23 



* 42X? 



25X34 



39X36 



45X46 



44X51 



Carnegie Mus. Field Mus. 

 2398 (type) 12168 



490 



"■310 



63-65 



295 



207 



84 



125 



22X21 



25X26 



25X27 



27X27 



67X? 



23X39 



40X44 



45X49 



41X46 



211 

 132 



" Estimated. ' Am. Mus. 2004. 



Lower jaw. — A lower jaw (Field Mus. 12166), found 

 on the same geologic level as the skulls but at some 

 distance, belongs to an aged individual. The crowns 

 of the incisors are almost worn away. The following 

 description and measurements are from Riggs (1912.1, 

 pp. 38, 39): 



The mandible is 10 millimeters shorter than would be required 

 to fit the skull, but the dentition matches closely. The molars 



have the strength necessary to oppose the massive upper 

 series; the canines and incisors, though not so massive as those 

 above, show such wear as would be expected in this form. The 

 canines are worn away diagonally at the point of contact with 

 the third upper incisor, but very little from contact with the 

 upper canines. There is a short diastema between canines and 

 premolars. The mandible as a whole is titanothere-like — deep 

 through the ramus, broad at the angle, concave in the tooth 

 line, and tapering toward the anterior extremity. The coronoid 

 is short and recurved at the tip. 



Lower jaw of Field Mus. 12166 



Millimeters 



Length, condyles to incisors 360 



Height, condyles above angle 168 



Length of molar-premolar series 215 



Length of molar series 130 



Length of crown of canine (estimated) 30 



Diameter of crown of canine 19 



Depth of ramus from base ofps 60 



Depth of ramus from base ofms 84 



THE MANTEOCERAS-DOLICHORHINUS GROUP (mANTE- 

 OCERAS, MESATIRHINUS, DOLICHORHINUS, SPHENO- 

 COELUS, METARHINUS, RHADINORHINUS) 



Stages, series, and subfamilies. — This second great 

 group of middle and upper Eocene titanotheres is 

 characterized by precocious horn swellings above the 

 eyes and many other featui'es in common. The single 

 specimen of this group (Eometarhinus) , discovered in 

 the Huerfano B ( = Bridger A) horizon of the Huerfano 

 formation of Colorado, is the sole known forerunner. 

 With this exception, this group is of much later geo- 

 logic appearance than the first group {Palaeosyops, 

 Telmatherium), being found in the upper levels of the 

 Bridger Basin, in the Washakie Basin, and in the lower 

 and middle levels of the Uinta Basin. The group 

 commenced to flourish in the Bridger and Washalde 

 regions during the period of the decline of the Palaeo- 

 syops phylum and survived it for a very long period, 

 but it was contemporaneous with the Telmatherium, 

 phylum. 



We find that this group radiates into four series, as 

 follows : 



Series included in the Manteoceras-Dolichorhinus group 



Besides the rudimentary horns there are very 

 numerous characters which tie the members of this 

 second group together and distinguish them from the 

 palaeosyopine group. These characters point indis- 

 putably to a common ancestor. An underlying unity 



of descent is at once observed in the accompanying 

 figures (fig. 302) of the four types of skulls included 

 in this group, which are all reduced to the same scale. 

 The four series are grouped into subfamilies and 

 genera as shown below. 



