THEORIES AS TO ORIGIN, ANCESTRY, AND ADAPTIVE RADIATION 



791 



observed as a very faint sessile, rounded swelling, on 

 the side of the face of a male Telmaiherium ultimum 

 of Uinta C 1. It would appear from this evidence, 

 which is very strong, if not absolutely conclusive, 

 that whereas in Manfeoceras and Dolichorhinus the 

 rudiments of horns were doubtless present at the 

 time of the deposition of Bridger B and are well 



rliinus) is entirely consistent with the retarded or 

 accelerated appearance of other characters, such as the 

 secondary cusps on the premolar teeth, as shown in 

 the above table. It is also consistent with the fact 

 that the Oligocene titanotheres divide into two great 

 branches, namely, the short-horned titanotheres, 

 including the Menodus and Brontops lines, and the 



Figure 712. — Evolution of the frontonasal horn swelling in members of the titanothere subfamilies 

 Manteoceratinae and Brontopinae 



A, Manteoceras manteoceras, middle Eocene; B, Proiiianotherium emarginntum, upper Eocene; C, calf stage of Oligocene titanothere (,fBron- 

 tops irachycephalus): D, young titanothere (Brontops hrachycephalus); E, Allops marshi; F, Brontops robustus. The horn swelling 

 is at the junction of the nasals and frontals, above the orbits. The frontal swellings grow forward, overlapping the nasals. In the 

 final stage the horn is far in front of the orbits and may be much compressed anteroposteriorly. 



developed at the base of Bridger C, they do not appear 

 in Palaeosyops until Bridger D, and not certainly in 

 TelmatJiermm until Uinta C 1. 



This indirect evidence of the retarded evolution of 

 the horns in certain of the titanotheres {Palaeosyops, 

 Telmatherium) and of their acceleration in other 

 Eocene lines (EometarMnus, Manteoceras, Mesati- 



long-horned titanotheres, including the Megacerops 

 and Brontotherium lines. It is, moreover, consistent 

 with the evidence repeatedly brought forward in this 

 monograph, that the chief difference between phyla 

 consists in the distinctive velocities of their character 

 evolution rather than in the presence or absence of 

 these characters. 



