EVOLUTION OF THE SKULL AND DENTITION OP OLIGOCENE TITAN OTHERES 



507 



Progressive pJiyletic characters. — Members of the 

 Allops phylum are known from the whole Titanothe- 

 rium zone. In general, the skull and teeth are inter- 

 mediate in character between the typical Brontops 

 and typical Menodus. The skull in males, originally 

 mesaticephalic, shows a marked progression toward 

 brachycephaly, the zygomatic indices rising from 64 

 to 76. The nasal bones in the males progressively 

 shorten as in members of the Brontops phylum. The 

 broadly trihedral basal section of the horns connects 

 these elements with Menodus rather than with Bron- 

 tops. The face is relatively longer than in Brontops 

 and more abbreviate than in Menodus. As in Bron- 

 tops the incisors are round topped with a formula of 

 2-1. One of the most distinctive features of Allops is 

 the transversely lanceolate form of the canine teeth 

 which enables us to connect A. vmlcotti with this series 

 rather than with the Menodus series. The opposite 

 grinding series are rectilinear, or nearly parallel, as in 

 Menodus. While the grinders approach those of 

 Menodus in having elongate or subhypsodont crowns 

 they are less dolichocephalic in proportion — that is, 

 the transverse diameters of the molar teeth slightly 

 exceed the anteroposterior diameters, whereas in 

 Menodus the reverse is the case. The dental index 

 is high — in males 46-47, in females 45-50. The 

 premolars exhibit pronounced external cingula as in 

 Menodus. 



Briefly, these animals resemble Menodus in the 

 trihedral shape of the horns and in numerous other 

 characters. They differ from the true Menodus in 

 the progressive abbreviation of the nasals, in the 

 brachycephaly of the zygomatic arches, in the reten- 

 tion of at least one pair of upper incisor teeth. Thus 

 they are provisionally assigned an intermediate 

 phyletic position. 



Several of the more advanced or upper-level speci- 

 mens of ^. serotinus were discovered in the overflow de- 

 posits of the upper Titanofherium beds rather than in 

 the main sandstone or channel deposits. This may 

 afi'ord some clue to the rarity of these crania. 



Characters of the genotype. — In describing the 

 genotype, Allops crassicornis , in 1887 Marsh charac- 

 terized it as a skull resembling in general that of 

 Menodus giganteus but as differing in the possession 

 of a single superior incisor tooth. The type possesses 

 a pair of well-developed second incisor teeth which 

 are always vestigial in Menodus but present in the 

 subgenus Menops. There are other still more im- 



portant differences, which will be enumerated in the 

 description of this species. 



Affinities of Allops. — Allops marshi has a very low 

 zygomatic index (64-69) in comparison with Allops 

 serotinus (72) and A. crassicornis (76), a fact which 

 suggested the theory that A. marshi may represent 

 the females of some other species such as B. dispar. 

 Some of the smaller skulls referred to Allops marshi 

 are with difficulty distinguishable from females of 

 Brontops hrachycephalus ; others approach B. dispar, 

 with which they agree in dental measurements (see 

 above); many are also strongly suggestive of Menodus 

 trigonoceras in the characters of the premolars and 

 molars and in the horns. All known skulls of A. 

 marshi are distinctly inferior in size to those of A. 

 serotinus. As shown by the detailed characters of 

 the skull and dentition and by the tables of measui'e- 

 ment, A. serotinus and A. crassicornis combine the 

 characteristics of Brontops and of Menodus in a 

 remarkable manner. They exhibit the characteristic 

 horn sections and distally squared nasals of Menodus, 

 the sharp premolar and molar cingula of Menodus, 

 combined with the broader skidl and broader grinding 

 teeth, expanded zygomata, shortened free nasals, 

 and retained incisors of Brontops. The form of the 

 canine also is more or less intermediate between the 

 conical canine of Brontops and the sharp-edged canine 

 of Menodus. The type and only known specimen of 

 Menops varians resembles Allops crassicornis in the 

 thick outwardly divergent horns, in the wide skull 

 (index 73), in the wide premolars and molars, in 

 the reduplicate tetartocone of p*. On the other hand 

 it approaches Menodus giganteus in the anteropos- 

 terior dimensions of the dentition and skull and in 

 the nasal and horn sections, so that it is placed in 

 the genus Menodus. 



These facts suggest the possibility that some of 

 the forms called Allops may represent a hybridization 

 between species of Brontops and of Menodus. 



Measurements, in millimeters, of lower jaws and teeth correlated 

 with and referred to Allops 



Posterior canine to hypoconulid of ma 



Pi-m3 



M ,-m:i 



Symphysis to condyle 



Depth below ms 



A. marshi, 

 Nat. Mus., 

 Gidley's skull 



320 

 335 

 225 

 375 

 94 



?A. walcotti, 



Nat. Mus., 



4247 



303 



298-1- 



200 



510 



108 



