474 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



A phylum somewhat intermediate between Brontops 

 and Menodus is Allops, from which members of the 

 Brontops phylum are distinguished by their retarded 

 tetartocones and by the fact that the horns in the 

 males are upright or slightly divergent, in contrast 

 with the depressed and widely divergent horns of 

 Allops. 



Retarded evolution of the Brontops premolars. — The 

 retarded molarization of the premolars is a conspicu- 



FiGXJEE 405. — Progressive evolution of the up- 

 per premolars in Brontotherium and its prede- 

 cessors 



A, Eotitanops borealis, Am. Mus. 14887, Wind Eiver; B, Palaeo- 

 syops paludosus, Am. Mus. 13032, lower Bridger (althougli 

 this genus is not in the Brontotherium series, it illustrates a 

 stage of premolar evolution); C, JRhadinorhinus diploconus, 

 Am. Mus, 1863, Uinta B; D, Brontotherium leidyi, Nat. 

 Mus. 4249, Chadron A; E, Brontotherium gigas, Am. Mus. 

 492, Chadron C. All one-half natural size. 



ous character of this phylum. The following facts 

 should be noted: 



1. The exact stage of evolution of the tetartocones 

 is partly obscured by the degree of wear, so that much 

 worn teeth appear simpler in structure than unworn 

 teeth, and the greatest degree of complication appears 

 in the intermediate stages of wear. 



2. All the specimens referred to the Brontops and 

 Diploclonus phyla show essentially similar premolar 



characters; in the primitive forms the tetartocone of 

 p* is a concave spur from the deuterocone, continu- 

 ous postero-internally with the internal cingulum; 

 the spur becomes more convex on the buccal side, 

 less concave on the lingual side, and gradually loses 

 its connection with the cingulum; the point of con- 

 striction between the deuterocone and tetartocone 

 moves forward so that the tetartocone enlarges at the 

 expense of the deuterocone. In all species of Brontops 



Figure 406. — Progressive evolution of the upper pre- 

 molars in Menodus and Brontops and their predecessors 



A, Eotitanops borealis. Am. Mus. 14887 (neotype). Wind Eiver; B, 

 Manteoccras manteoceras. Am. Mus. 12683, upper Bridger (Telmatherium 

 cultridens illustrates this stage even better); C, Biplacodon elatus, YalQ 

 Mus. 11180, Uinta C; D, Brontops irachycephalus, Nat. Mus. 4258 (type), 

 Chadron A; E, Menodus giganteus, Am. Mus. 505, Chadron C. All one- 

 half natural size. 



the tetartocone never appears entirely distinct from 

 the deuterocone, as it does in Brontotherium. 



3. Within the species B. dispar there is considerable 

 range of evolution in the progressive development of 

 the tetartocone, possibly due to the crania having 

 been found on different levels. 



4. An interesting fact is that occasionally there are 

 noticeable differences in the tetartocones on opposite 

 sides of the same individual. 



