EVOLUTION OF THE SKULL AND DENTITION OF OLIGOCENE TITANOTHERES 



499 



rohustus. The hind limbs mounted with this skeleton 

 belong to other individuals. 



On the whole the evidence indicates that the 

 mounted skeleton is not a female brontothere but is a 

 member of the Brontopinae and probably Brontops 

 rohustus. 



Diploclonus Marsh 



{"Allops" Marsh, Osborn, 1902) 



Plates XXXVII, CIV-CX; text figures 185, 



187, 196, 200, 201, 375, 389, 391, 394, 397, 



409, 426-428, 630, 639 

 [For original description and type references see p. 227. 

 For skeletal characters see p. 675] 



Generic characters. — A phylum col- 

 lateral to other Brontops phyla, appar- 

 ently distinguished by the budding off 

 of a second hornlet on the inner side of 

 the horns and hence loiown as Diplo- 

 clonus; also by the narrow and elon- 

 gate nasals in the middle members of 

 the series; hornlets rising from the 

 inner faces of the horns. 



Incisors 2—1. Premolars about as 

 in B. dispar, but with external cingula 

 becoming obsolete. Nasals progres- 

 sively abbreviating. Bridge over in- 

 fraorbital foramen rather sharp and 

 narrow. Progressive brachycephaly ; 

 also backward prolongation of the 

 temporal fossae. Other characters as 

 in Brontops. 



A tendency to give off an internal 

 branch, knob, or "hornlet" (fig. 427) 

 on the inner and anterior faces of the 

 main horns is observed in a number 

 of titanothere skulls which otherwise 

 show strong affinities to the Brontops 

 phylum and especially to the species 

 B. dispar. Marsh regarded this inter- 

 nal hornlet as of so much importance 

 that he assigned it generic rank in 

 proposing the genus Diploclonus, the 

 genotype of which is the species Diplo- 

 clonus amplus. The presence of a small 

 accessory horn swelling has also been 

 observed in certain skulls of Menodus 

 and of Brontotherium. It is doubtful, 

 therefore, whether this hornlet is of 

 generic value. D. amplus and D. tyleri 

 resemble B. rohustus in many features but may 

 represent a distinct subphylum. Subsequently the 

 same character was observed by Osborn in two skulls 

 in the American Museum (Nos. 1476, 1081) which 

 were made the type and paratype respectively of the 

 species Diploclonus {" Megacerops") hicornutus (fig. 



a fine skeleton with skull was obtained by the Amherst 

 Museum and described by Lull as a third species Dip- 

 loclonus {" Megacerops") tyleri (Amherst Mus. 327). 

 The sum of progressive characters is as follows: 

 (1) Hornlets on the inner sides of the horns; (2) in- 

 creasing brachycephaly; (3) gradual or retarded 

 evolution of premolars, p* being the most retarded 

 tooth. The sum of retrogressive characters is as 



A B 



Figure 427. — Sections and contours of skull of Diploclonus amplus 

 A, Nat. Mus. 4710. In this supposed female the horns are relatively short, the connecting crest well devel- 

 oped, horn section trihedral, nasals broad, zygomata slender, B, Yale Mus. 12015a (type); a male, with 

 horns relatively long, directed forward and placed far in front of the orhits, basal horn sections trihedral, 

 connecting crest high, zygomata well expanded, nasals small and narrow. One seventh natural size. 



follows: (1) Relative abbreviation of the premolar- 

 molar series, the index in D. hicornutus being 45, which 

 is less than that in D. amplus; (2) apparent reduction 

 or loss of median incisors. 



A doubtful genus and phylum. — There is no question 



whatever of the affinities of these animals to Brontops 

 426), the latter now referred to D. tyleri; and finally j and of their substantial proximity to this genus. 



