516 



TITANOTHERES OP ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



from in front the inferior contour of the horns is 

 straight, as in M. giganteus; the angles of the basal 

 portion are more sharply trihedral or defined than in 

 the type of A. crassicornis. As seen from above we 

 observe a decided midtemporal as well as supraciliary 

 overhang of the supratemporal crest, as in the type 

 of A. walcotti (Nat. Mus. 4260). The supratemporal 

 crests exhibit, in fact, a prominent flare or projection 

 a short distance behind the orbits. The occiput is 

 deeply excavated, but the occipital pillars are rela- 



than in the large type Menodus" giganteus. The distinc- 

 tive anteroposteriorly compressed form of canine is well 

 represented in Plate CXXXII, C. The postero- 

 internal cusps of the premolars are much less devel- 

 oped than in A. crassicornis or in M. giganteus — in 

 fact, in p* the tetartocone is a feebly developed ridge, 

 and the deuterocone forms the most internal portion 

 of the crown; in other words, this tooth is in a primi- 

 tive stage of evolution. The tetartocone of p^ is a 

 mere spur. The hypocone of m' is an elevated cin- 



m 



A 



B 



Figure 433. — Sections and contours of skulls of AUops serotinus and A. crassicornis 



A, AUops serotinus, Nat. Mus. 4231 (type); upper levels of Chadron B; stout horns directed outward (although here represented as vertical) with a well- 

 defined trihedral basal section (as in Menodus), nasals short and wide, zygomata relatively slender. B, A. serotinus, Nat. Mus. 4938; lower levels 

 of Chadron C; horns directed outward and roundly trihedral in basal section, nasals short, thick, and very wide, zygomata expanded. C, A. 

 crassicornis, Nat. Mus. 4289 (type); lower levels of Chadron 0; horns stout and very thick, basal section roundly trihedi'al, zygomata not much 

 expanded. All one-eighth natural size. 



tively light and are surmounted by a simple rugose 

 flare. As in Brontops and Menodus the occiput is 

 decidedly produced behind the zygomata. As in 

 Menodus the zygomatic arches are relatively deep 

 and narrow, and the parietal vertex is also relatively 

 more constricted than in BrontotJierium and Megacerops. 

 Dentition. — -The dental characters of the type are 

 highly distinctive. A large single incisor persists on 

 each side, relatively more robust than that in A. 

 crassicornis. The canine crowns measure 41 millime- 

 ters anteroposteriorly and are thus slightly smaller 



gule, contrasting with the cone which is so distinctive 

 a feature of Menodus. 



In its premolar evolution A. serotinus is more re- 

 tarded than A. crassicornis. 



Female .skull Nat. Mus. 3151.— The skull (PI. 

 CXVII, D) found not far from the' type of A. serotinus 

 is believed to be a female of the same species. It 

 differs from the type in several structural characters 

 which may be attributed partly to the differences of 

 sex, as follows: (1) It is of smaller size; (2) the horns 

 are more sessile, resembling those of A. marshi; (3) the 



