EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITA.NOTHERES 



421 



origin, because fossils are rarely found in the shales 

 which alternate with these ledges (Riggs, 1912.1, p. 24). 

 Skulls are often embedded in the semigravelly layers 

 and have their narial or orbital cavities filled with 

 pebbles which could be carried only by rapidly 

 flowing water. Another evidence of stream action 

 lies in the complete dissociation of the various 

 skeletal elements; seldom are the lower jaws associated 

 with the skulls, or so many as two vertebrae found 

 articulated. In the exceptional instances where the 

 whole skeletons are but little disturbed they are found 

 embedded in the fine-grained homogeneous sandstone 

 apparently deposited in more quiet water, such as 

 deep pools or eddies. 



This prevailing mode of occurrence supports Os- 

 born's theory that these animals were of semiaquatic 

 or fluviatile habits, as indicated by tne name given to 

 the type species of the genus, M. fluviatilis. 



Generic characters. — Small titanotheres, basilar 

 length 355 to 415 millimeters. Persistently mesatice- 

 phalic; postcanine diastema short; orbits very promi- 

 nent, with projecting infraorbital shelves; nasals 

 expanding distally; narial opening deeply recessed 

 at sides; premaxillary symphysis greatly elongated; 

 snout moderately broad; rudimentary horns on fronto- 

 nasal suture; sagittal crest high and thin. Incisor 

 teeth small, cingulate; canine teeth small, pointed, 

 recurved; grinding teeth subhypsodont; pz'emolars 

 rather progressive; hypocone of m^ present or absent; 

 hypoconulid of ms small, conic. 



Historical notes. — The type species of this genus was 

 recognized by Osborn (1908.318) from a specimen 

 found in Uinta B 1 , which had been confused previously 

 with Mesatirhinus megarhinus. This is a very small 

 animal, perhaps the primitive member of the series. 

 At the same time Osborn recognized in Washakie B a 

 second species, M. earlei, distinguished from M. fluvia- 

 tilis by its much greater size and the lesser prominence 

 of the orbits. After the successful expedition of 

 1910 in the Uinta Basin, Riggs (1912.1) added M. 

 cristatus, an animal of intermediate size but of the 

 same proportions as M. fluviatilis, also M. ripa?-ius, 

 an animal of larger size with an apparently longer 

 and narrower skull. 



Osborn placed in the genus MefarJiinus the species 

 M. diploconus, which Riggs (1912.1) on excellent 

 groimds removed to the new genus RhadinorMnus. 

 It is possible that the Palaeosyops Junius of Leidy, 

 from Bridger B, a very diminutive form, represents 

 an ancestral form of this metarhine phylum. 



Is Metarhinus diphyleticf — Riggs (1912.1, p. 27) 

 regards the genus as including two phyla, one contain- 

 ing M. fluviatilis and M. riparius, which were more 

 primitive and had longer heads and larger canines, 

 the other containing M. earlei and M. cristatus, 

 which were relatively shortheaded and had somewhat 

 more progressive teeth. The restudy of these forms 



by cephalic indices partly sustains Riggs's opinion, 

 but indices can be depended upon only when a con- 

 siderable number of skulls can be measured, because 

 the effect of crushing is deceiving. The indices 

 actually taken are as follows: M. earlei, 63, 61, 60, 

 60, 60; M. cristatus, 60 (estimated); M. fluviatilis, 58, 

 56; M. riparius, 55, 51. 



It would appear from these indices that M. earlei 

 contains the forms with broadest heads and that 

 M. riparius contains the forms with narrowest heads. 

 There are also other characters which may divide 

 these animals into two phyla, as shown below. 



Phyla of Metarhinus 



The synopsis of these species in chronologic order 

 is as follows: 



Metarhinus fluviaiilis Oahorn. Middle of Uinta B 1; skull 

 small, moderately broad (length 352 mm., breadth 205; cephalic 

 index 56-58); a hypocone on m^. 



Metarhinus earlei Osborn. Summit of Uinta B 1 and Wash- 

 akie B; skull somewhat larger (length 405 mm., breadth 255, 

 or 338:245); somewhat broader (cephalic index 60-63); 

 rudiments of a secondary palate; no trace of hypocone. 



Metarhinus cristatus Riggs. Lower level of Uinta B 1; 

 type skull of intermediate size (length 385 mm., breadth 240; 

 cephalix index 60); hypocone present, cingulate; similar to 

 M. fluviatilis. 



Metarhinus riparius Riggs. Summit of Uinta B 1; skull 

 larger (length 406 mm., breadth 210, or 406 : 210; cephalic 

 index 51-55, estimated); apparently longer and narrower; 

 a hypocone on m^ 



Metarhinus fluviatilis Osborn 



Plates LII, LXXI, LXXIV; text figures 123, 341, 356-358, 404 



[For original description and type references see p. 183. For skeletal cliaracters 

 see p. 644] 



Type locality and geologic Jiorizon. — White River, 

 Uinta Basin, Utah; Metarhinus zone (Uinta B 1); 

 abundant. 



Specific characters. — Relatively short, broad skull, 

 length 352 millimeters, breadth 205, or 355: 200; cepha- 

 lic index 56 to 58. Eye socket small; circumorbital 

 ridges prominent; premaxillary symphysis elongate; 

 basicranial region short; sagittal crest high and 

 prominent. Grinding teeth subhypsodont, m^ with 

 a rudimentary "cingulum hypocone" in the type. 



Geologic distribution. — Uinta B 1 is the horizon of 

 the type specimen (Am. Mus. 1500) and of the 

 referred specimen (Am. Mus. 1877). Similar but 

 somewhat more progressive forms showing the same 



