580 



MTANOTHERES of ancient WYOMING, DAKOTA, AND NEBRASKA 



anteroposterior. It is difficult to imagine of what 

 service these small incisors could have been to the 

 animal and why they should have been preserved in 

 this phylum while they have degenerated in the 

 others. But the fact of their persistence proves that 

 they were used in some manner, at least in the males. 

 The canines have degenerated; they are smaller than 

 those of B. gigas and very much smaller than those of 

 B. ramosum. The premolars have less distinct internal 

 cones than in B. ramosum and are seen to correspond 



M^ 



A 



B 



Figure 481. — Sections and contours of skull of Brontotherium plaiyceras 



A, Harvard Mus. (type); B, Am. Mus. 1448 (neotype). One-ninth natural size. In tliis species the very long 

 horns have a much flattened section and are placed far in front of the orbits. The free portion of the nasals is 

 almost vestigial, and the zygomatic expansions are very massive. 



rather with those of B. gigas. The cingula on the 

 posterior molars are obsolete. 



STcuU. — As we have seen in the type the elevation 

 of the connecting crest between the horns is so great 

 that it now connects them at the base, forming a 

 broad, continuous crest (PI. CLXXXVII) subcon- 

 cave posteriorly, with sharp edges. 



The sections of the horns in the American Museum 

 skull (No. 1448, fig. 481, B) were taken just above 

 the crest. They are extremely broadened, with the 



anterior faces slightly convex and the posterior faces 

 plane or slightly concave. The posterior concavity 

 of the horns, observed in the Harvard type specimen 

 (fig. 481), is here rendered plane by crushing. This 

 plane section is carried nearly to the summits, which 

 do not expand as in B. ramosum. A comparison of 

 the sagittal sections with those of the previously 

 described species also illustrates the extreme thin- 

 ning, anteroposteriorly, and the vertical face of the 

 connecting crest. The fact that this crest and the 

 horns incline forward is probably 

 due to vertical crushing. 



The nasals are reduced to 20 

 millimeters, one-half the length 

 observed in B. ramosum and B. 

 curtum. The entire outer edges of 

 the horns are now composed of 

 the sharp external ridges, which 

 reach from the summit to the ante- 

 rior superior rim of the orbit, where 

 they are grooved horizontally by 

 the lacrimal ducts. The comple- 

 tion of this sharp outer face is the 

 fulfilment of a growth tendency 

 which we have seen in various de- 

 grees of development from 2 to 

 many inches in length in preced- 

 ing stages. 



The entire anterior portion of 

 the neotype skull is distorted by 

 pressure; the posterior portions are 

 entirely wanting and have been re- 

 stored in plaster. The external au- 

 ditory meatus was entirely closed 

 by a deep union of the postglenoid 

 and post-tympanic processes. The 

 enormous zygomata were fortu- 

 nately preserved, especially that 

 upon the right side, which is accu- 

 rately represented in the section. 

 The mass of this buccal process 

 is as great as in B. curtum and 

 apparently exceeds that of B. 

 ramosum. This section and the 

 less progressive structure of the 

 premolars furnish two reasons for regarding B. plaiy- 

 ceras as derived from the B. gigas datum and B. 

 curtum types rather than from the B. ramosum type. 

 According to this view B. ramosum would represent a 

 contemporary or collateral species rather than one in 

 the direct line of ascent. 



Additional observations on the measurements of Bron- 

 totherium curtum and B. plaiyceras. — The five male 

 skulls referred to B. curium exhibit the following range 

 in measurements as compared with B. plaiyceras: 



