500 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



From the materials at hand, which are hmited to four 

 or five skulls at present, it is doubtful whether they 

 should be separated as a distinct phylum of generic 

 value. The question turns on whether the hornlet 

 is a sport character or a permanently progressive 

 character, which can be settled only by more extensive 

 material than is now available. On the whole, it 

 seems to be wisest at present to regard these species 

 as a subphylum collateral with the- main phyla of 

 B. dispar and B. robustus. 



Affinities with Brontops dispar and Allops marsTii. — • 

 Among the chief resemblances to Brontops dispar are 

 (1) the marked brachycephaly, especially in the widely 

 expanding buccal processes of the zygomata, empha- 

 sized most distinctly in D. tyleri and D. amplus; (2) the 

 marked backward prolongation of the occiput behind 

 the widest portion of the zygomatic arches, which dis- 

 tinguishes these skulls at once from those of Allops 

 marshi and B. robustus and relates them to B. dispar; 

 (3) the short horns with rounded summits which par- 

 take of the general characters of those of B. dispar, 

 although a tendency to broaden and flatten becomes 

 marked in D. tyleri and extreme in D. amplus. The 

 nasals show progressive abbreviation: they are elon- 

 gate in D. hicornutus, more abbreviate in D. tyleri, and 

 extremely abbreviate in D. amplus. Features of 

 difference from B. dispar are not only the internal 

 hornlets upon the horns but the somewhat more 

 marked development of the connecting crest between 

 the horns. 



Unfortunately the records as to geologic distribution 

 are very incomplete. The geologic level of the least 

 progressive stage, D. hicornutus, is not recorded. The 

 level of the intermediate stage, D. tyleri, is recorded 

 as 35 feet above the Pierre shale, 200 feet below the 

 summit of the Titanotherium zone; this would place 



this animal in lower B or even in A. The level of the 

 most progressive species, D. amplus, is not recorded. 



Distinctive characters of the species of Diploclonus. — 

 This is not a monophyletic series, like Menodus or 

 Brontotherium; it is diphyletic. The skulls at once 

 seem to be distinguished from those of Allops marshi 

 and Brontops rohustus by the great backward extension 

 of the occipital region behind the zygomata. The pit 

 in the center of the vertex relates them to other species 

 of Brontops. The progressive broadening of the 

 zygomata closely parallels that of the other collateral 

 phyla. 



D. hicornutus (Osborn) is the most primitive form, with 

 relatively long and narrow nasals, rounded and more erect 

 horns, least expansion of the zygomatic arches, and least 

 depression of the angular border of the jaw. 



jD. selwynianus (Cope) is a little-known animal from the Cy- 

 press Hills, Saskatchewan, represented only by the nasal bones, 

 which are intermediate in size between those of D. hicornutus 

 and D. tyleri; they resemble the nasals of D. hicornutus more 

 closely in their narrow, elongate, and laterally decurved form. 



D. tyleri (Lull) is an intermediate stage of evolution, char- 

 acterized by relative abbreviation of the nasals, more anterior 

 position of the horns, greater prominence of the hornlets, more 

 widely arched zygomata, depression of the lower angular region 

 of the jaw. This animal is a more pronounced development of 

 the D. hicornutus type, the internal hornlets being larger, the 

 horns much broader and more decidedly projecting forward. 

 The geologic level of this animal is said to be 50 feet above the 

 Pierre shale. It is probably from the middle levels, B. 



D. amplus Marsh is distinguished by very short and obtuse 

 nasals and a prominent internal hornlet on the horns, horns 

 broadly divergent and compressed anteroposteriorly, excessively 

 wide zygomatic arches, stout, recurved canines. The features 

 of this progression as seen in the superior view of the skulls are 

 displayed in Plate CVIII, A, and in Figure 391. This robust 

 animal shows an extreme development of the characteristics of 

 this subphylum. Massive skuH, very broad zygomatic arches, 

 very short horns, with a decided development of the internal 

 hornlets. Probably belongs on the levels of Chadron of C. 



