EVOLUTION OF THE SKULL AND DENTITION OP OLIGOCENE TITANOTHERES 



503 



Measurements oi Di-ploclonus tyleri and Menodus irigonoceras, 

 in millimeters 



This agreement with Menodus irigonoceras, taken 

 in connection with the relatively slender manus and 

 pes of the type of D. tyleri as compared with B. 

 roiustus, constitutes one of the many facts which 

 suggest the possibility of occasional hybridization 

 among the genera Bron- 

 tops, Menodus, and AUops 

 (W. K. Gregory). 



Description of the 

 type. — Lull's specific defi- 

 nition may be cited in full 

 (Lull, 1905.1, p. 445): 



Horns well in front of orbits, 

 directed somewhat forward 

 and outward, an elongate oval 

 in basal section with the long 



axes in line, rounded oval at the summit. Hornlets quite 

 conspicuous, on the inner face of the horns midway between 

 tlie base and summit. Connecting crest low and inconspicuous. 

 Nasals broad, well rounded in front, and but slightly arched 

 beneath. Zygomata expanded and deep, with a well-rounded 

 outer face. Dentition: Superior incisors represented by the 

 deep and well-defined median alveoli and by the lateral teeth, 

 which remain in place and which have hemispherical crowns 

 which show little sign of wear. The canines are 

 lanceolate, with a well-developed postero-internal 

 cingulum. There is a short diastema in front of, 

 and a longer one behind, the canine. Premolars 

 with a smooth internal cingulum, less pronounced 

 in the middle of the tooth and with no external 

 cingulum. The deuterocone is well developed, 

 while the tetartocone, especially of premolar 4, 

 is inconspicuous. 



The jaw is deep and robust, with the alveoli 

 of two incisors, probably of the second and third, 

 deep and distinct. There is no space between 

 the lateral incisors and the canine, though be- 

 tween the two median alveoli a considerable gap 

 occurs. There seems to have been a small 

 diastema behind the lower canines, which are 

 lanceolate, though with a less prominent cingulum and not so 

 strongly recurved as the upper ones. 



The same author observes that the creature most 

 nearly resembles D. bicornutus (Osborn) and D. amplus 

 Marsh, having certain characters suggestive of each; 

 but there are enough important differences to render 

 it distinct and to warrant the erection of a new species 

 for its reception. The dental formula is If, C\, P|, 

 M|; the two median superior and all of the lower 

 incisors are represented by deep inclosed alveoli, as 

 101959— 29— VOL 1 35 



if the teeth had dropped out after death. Toward 

 the base the horns are oval to triquetrous in section; 

 toward the summit they become rounder and rough- 

 ened at the extremities. "It would seem," observes 

 Lull, "from the similarity of the roughened patches 

 to those on the rhinoceros nasals, as though the entire 

 prominence had been clothed with skin, with two 

 rhinoceros-like horns, a larger one at the apex and 

 a smaller one on the summit of the hornlet." 



Figure 428. — Lower jaws of Diploclonus bicornutus and D. tyleri 



, Diploclonus bicornutus, Am. Mus. 1476 (type); animal very old and hence the angle is broad and 

 prominent (compare the aged type of M. torvus, fig. 437, A). Canines stout and conical, external 

 cingula not sharply separated from the eotoloph, B, D. tyleri, Amherst Mus. 327 (type); broadly 

 resembles S. robusius. A diastema in front of pi. One-fifth natural size. 



Characters of referred specimen (Am. Mus. 1081). — 

 This specimen was employed by Osborn as a paratype 

 of D. hicornutus. Skull broad, index 85 ; tooth row elon- 

 gate (367 mm.) ; index 50. Condyles to incisive border 

 710 millimeters. Horns anteriorly placed and directed 

 obliquely forward. It is important to note that the 

 plaster restoration of the nasals (Pis. CV, CVII) and 

 of the anterior part of the zygomata prevents a true 

 determination of the characters of these parts of the 

 skull. This specimen is also of advanced age and 



