EVOLUTION OP THE SKULL AND DENTITION OF OLIGOCENE TITANOTHERES 



459 



Allops walcotti 



Figure 389. — Skulls of the menodontine group 



Side view. One-twelfth natural size. 



Subfamily Brontopinae: A, Brontops bracJiycepMlus, °, a very small and primitive member of the Brontops phylum, with mcipient 

 horn swellings, long nasals, and slender canines; Chadron A. B, Brontops dispar, an intermediate stage with moderately developed 

 horns; Chadron B. C, Brontops robustus, final stage of this phylum; a very massive brachycephalic skull with stout forward 

 directed horns, short, thick nasals, short, heavy occiput, and expanded zygomata; canines short and thick; Chadron C. D, Viplo- 

 clonus bicornutus, referred to this genus partly because of the accessory horn swelling (h'). The skull contour suggests that of 

 Menodus. E, Diploclonus amplus, a massive short skull recalling Brontops robustus but possessing an accessory horn swellmg and 

 verv short nasals. Occiput long. 



Subfamily Menodonfinae: F, Allops walcotti, a primitive dolichocephalic form, supposed to be ancestral to the Allops phylum; Chadron 

 A G, Allops marshi, in general contour intermediate between Brontops dispar (B) and Menodus (H), thought to be related to 

 Allops serotinus and Allops crassicornis. H, Menodus giganteus, latest stage of the Menodus phylum; uolichocepnalic, horns trihedral 

 in basal section, nasals long, zygomata not expanded, canme long, cheek teeth with sharp external cingula; Chadron O. 



In all members of this group the skull top in side view is deeply concave. The cheek teeth almost always have sharp external cingula. 



