504 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



shows many senescent characters. The teeth are 

 extremely worn and reveal nothing decisive; the 

 cingula of the premolars are smooth and worn down. 

 The horn tips are extremely rugose, and the internal 

 hornlets are well marked (fig. 426, B). The occipital 

 vertex is rugose, with deep paired indentations and 

 knobs. The zygomata are greatly swollen trans- 

 versely. As in many aged specimens of Brontops, 

 a single large incisor is preserved in either premaxUla; 

 the superior incisive formula was thus 2-1. 



Diploclonus amplus Marsh, 1890 



{"Allops" amplus, Osborn, 1902) 



Plates CVIII-CX; text figures 187, 376, 389, 391, 394, 409, 

 427 



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



Geologic Jiorizon. — Titanotherium zone, level not 

 recorded. 



Specific characters. — Skull extremely brachycephalic, 

 zygomatic-basilar index 91. Nasals greatly abbre- 

 viated and narrow, horns broadly oval in basal section, 

 directed upward and outward, with conspicuous 

 internal hornlets. Grinding series relatively reduced, 

 length 302 millimeters, index 45. Upward flexure of 

 premolar series anteriorly pronounced. Zygomata 

 extremely broad with convex buccal expansions. 



General characters. — The nasofrontal section of the 

 type skull first suggested comparison with a form 

 transitional between Brontotheriwn gigas and B. 

 curtum, but numerous other characters forbid such a 

 phyletic reference, especially the long-pointed canines, 

 40 millimeters (the type was a male), the presence of 

 only one superior incisor on each side, the retarded 

 development of the tetartocones of the premolars, 

 the rounded tips of the horns. All these characters 

 remove the animal from relationship with Bronto- 

 therium, Menodus, or Megacerops and indicate its 

 affinity to Brontops; this is confirmed by more 

 searching study of the character of this type, but 

 especially by the existence of the more primitive and 

 transitional forms D. hicornutus and D. tyleri above 

 described. 



Diploclonus amplus is by far the most progressive 

 species in this phylum, as demonstrated by (1) the 

 extreme abbreviation of its nasals, (2) the strong 

 development of the connecting crests between the 

 horns, (3) the greater prominence of the internal 

 hornlets, and (4) the transverse-oval expansion of the 

 basal horn section, which has now reached an extreme 

 stage, parallel to that observed in Brontops rohustus. 

 This species, however, is readily distinguished from 

 Brontops rohustus, in common with other members of 

 the phylum to which it belongs, by the marked 

 backward elongation of the occipital region behind 

 the zygomatic arches. 



This specimen tends to confirm the hypothesis that 

 the species grouped under Diploclonus formed one or 

 more subphyla parallel with Brontops. 



Materials. — The type skull of D. amplus in the 

 Yale Museum (No. 12015a) is undoubtedly a male. 

 A supposed female skull is found in Nat. Mus. 4710. 

 The type is the only specimen which we have yet 

 seen that certainly belongs to this species.. 



Observations on the measurements of Diploclonus 

 amplus. — The type and only known male skull of this 

 species is very brachycephalic, although crushing may 

 have contributed to the extremely high zygomatic 

 index (91). 



The male skull approaches the paratype of Allops 

 marshi in the anteroposterior measurements of the 

 dentition, but in its general conformation it rather 

 suggests B. rohustus. The nasals are reduced in size. 



Measurements of Diploclonus amplus and Allops marshi, in 

 millimeters 



Pi-m3 



Pi-p' 



M'-m3 



Pmx to condyles 

 Zygomatic index 



D. amplus, 

 Yale Mus. 

 12016a (type) 



(?)302 



133 



207 



675 



(?)91 



A. marslii. 

 Am. Mus. 

 1445 (para- 

 type) 



335 

 135 

 203 

 675 

 69 



Detailed description of the type. — As seen from above 

 the skull is broad and short in proportion, the index, 

 91, expressing its marked brachycephaly. We are 

 struck by the strong divergence of the horns, a feature 

 which is probably intensified by vertical crushing. 

 They are slightly convex on the anterior surface, with 

 very prominent external ridges. They are united by a 

 very deep connecting crest, as exhibited in the section. 

 The prominent tuberosity or hornlet employed by 

 Marsh as a generic character is 14 centimeters below 

 the tip of the horns; it is especially well developed on 

 the left side, measuring 37 millimeters anteroposteriorly 

 and 35 transversely. A very distinctive feature of the 

 horn is the sharpness of the preorbital crest. As seen 

 from above (PI. CVIII, A) the zygomata project 

 widely outward. A slightly distorted section of the 

 buccal processes is shown in the diagram. The upper 

 surfaces are somewhat flattened. In the middle of the 

 vertex is a shght tuberosity which probably indicates 

 the vestige of the pit so characteristic of members of 

 the Brontops phylum. The occiput is deeply exca- 

 vated with stout lateral pillars and rugose summits. 

 It is considerably produced backward behind the zygo- 

 mata. In inferior aspect the posterior nares are seen 

 to open just opposite the third molar. The pterygoid 

 wings of the aHsphenoid are short, rugose, flaring 

 strongly outward; the presphenoid is keeled by the 



