502 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



Similar anterior tuberosities, or hornlets, are observed 

 in certain male skulls of M. giganteus (Am. Mus. 

 505). As seen ftom above (fig. 444) the parietal 

 vertex is moderately broad. A very distinctive 

 feature is a sharp preorbital malar bridge. There is 

 considerable expansion behind the orbits, as in B. 

 dispar; as in all aged individuals, the supratemporal 

 crests are sharp and overhanging. The summits of 

 the occipital pillars are rugose and greatly expanded. 

 In lateral view (PI. CIV) the postglenoid process is 

 seen to be relatively narrow and exhibits a short 

 surface of contact with the post-tympanic. On the 

 palatal surface of the skull the vomer apparently does 

 not extend back to overlap the parasphenoid. There 

 is only a slight basisphenoid rugosity. 



This skull shows group resemblances to Menodus 

 trigonoceras as follows: (1) Nasals long, set very high; 



(2) orbit very large; (3) tooth row very long (dental 

 index 50, 46-51 in M. giganteus); (4) zygomata not 

 expanded and in side view pitching sharply downward 

 and forward; (5) lower jaw resembling that of Menodus 

 in the well-developed chin and depressed angle. 



Dentition. — The incisor alveoli are not well defined. 

 The canines are much worn; they exhibit crowns of 

 rounded form. The absence of external cingula on 

 the premolars may be due in part to the extreme wear. 

 The internal cingula are well developed. The tetarto- 

 cone developments on the premolars are approxi- 

 mately of the same stage as in B. dispar. In m^ the 

 hypocone is an elevated part of the cingulum. 



Summary. — This skull may be that of an old male, 

 affording an example of a progressive oft'shoot of one 

 of the Brontops lower beds phyla. 



Diploclonus selwynianus (Cope) 



{Menodus s^elwynianus Cope, 1889; " Megacerops?" selwynianus 

 Osborn, 1902) 



Text figure 185 

 [For original description and type references see p. 225] 



Type locality and geologic horizon. — Swift Current 

 Creek, Saskatchewan; Titanotherium zone. 



Specific characters. — Nasals long (free length about 

 115 mm.), narrow (free width 75 mm.), abruptly 

 rounded in front and sharply decurved at the sides, 

 with a deep longitudinal inferior cavity. 



The chief ground for referring the uniquely narrow 

 nasals of the type of this species to Diploclonus is the 

 resemblance they bear to those of Diploclonus bicornu- 

 tus, which is closer than to that of any other form. 

 The resemblances in the nasal bones consist in (1) the 

 narrowness, (2) the deep longitudinal inferior cavity, 



(3) the decurved sides, and (4) the massive and de- 

 curved tips. 



Cope's description of this specimen from the Swift 

 Current Creek region is cited in full elsewhere (p. 226). 

 To this description may be added the following ob- 

 servations. The inferior view (fig. 185) exhibits the 

 paired cavities, or frontal antrum, at the junction of 



the nasals, frontals, and maxillaries; the suture be- 

 tween the nasal bones has entirely disappeared. 

 These nasals are very characteristic and quite dif- 

 ferent from those of Menodus or Megacerops colo- 

 radensis. Lambe describes the type as follows 

 (1908.1, p. 47): 



The coossified nasal bones of one individual constitute the 

 type of this species. They are long and narrow, abruptly 

 rounded in front, and bent downward at the sides. The lower 

 surface is deeply excavated in a longitudinal direction. 



Diploclonus tyleri (Lull) 



{Megacerops tyleri Lull, 1905) 



Plates XXXVII, CV-CVIII; text figures 200, 201, 397, 409, 

 426, 428, 630, 639 



[For original description and type references see p. 234. For slteletal characters 

 see p. 675] 



Geologic horizon. — Recorded as 35 feet above the 

 Pierre Cretaceous, or 165 feet below the summit of the 

 Titanotherium zone. Regarded by Lull as in the upper 

 part of the lower Titanotherium zone, but more 

 probably belonging in the level of the middle beds. 



Specific characters. — Brachycephalic. Zygomata 

 widely arching. Nasals abbreviated or broad; free 

 length, 140 millimeters. Grinding series, 363. Horns 

 shifted forward; distinct hornlets on the inner sur- 

 faces; horn section at the base trihedral to trans- 

 versely oval, with sharp outer angle. Canines stout, 

 recurved. Superior incisors 2-1. 



General characters. — This animal is certainly a 

 member of a collateral branch of the B. dispar series, 

 agreeing with the aged specunens of the typical 

 B. dispar even more closely than the type of D. 

 bicornutus. The species appears to represent a stage 

 of phyletic evolution intermediate between that of 

 the more primitive D. bicornutus and the more pro- 

 gressive D. amplus. This conclusion, however, awaits 

 confirmation by additional evidence. 



Materials. — The type of this specific stage or 

 ascending mutation is the fine skeleton and anterior 

 portion of the skull in the Amherst collection (No. 327) . 

 Apparently belongmg to the same stage is the finely 

 preserved skull and jaws (Am. Mus. 1081) originally 

 described by Osborn as a paratype of D. bicornutus 

 but presumably representing a more recent or pro- 

 gressive stage of evolution. 



Observations on the measurements of Diploclonus 

 tyleri. — The two skulls referred to Diploclonus tyleri 

 differ from the type of D. bicornutus in having larger 

 molars and more massive horns. They differ from 

 the type of D. amplus in having larger molars. The 

 type skull and jaw in side and top views approaches 

 that of Brontops robustus, except that it has a well- 

 developed pair of accessory horn swellings. The 

 characters of the incisors, canines, postcanine diaste- 

 mata, and premolars further suggest affinity with 

 B. robustus; but the anteroposterior measurements 

 of the dentition differ from those of Brontops and agree 

 with those of Menodus trigonoceras, as shown below: 



