EVOLUTION OF THE SKULL AND DENTITION OF OLIGOCENE TITANOTHERES 



537 



Measurements of Menodus varians 



Millimeters 



Length of skull, vertex to tip of nasals 695 



Basilar length, occipital condyles to premaxillary 760 



Transverse width across zygomata 555 



Molar-premolar series 410 



True molar series 255 



Premolar series 155 



Canine crowns, anteroposterior diameter 35 



Free length of nasals 105 



Free width of nasals 140 



Dentition. — The alveoli for two vestigial iacisors 

 upon each side persist, with a narrow median diastema 

 between them. There is a vestige of a third alveolus 

 on one side. The alveoli are exceptionally shallow, 

 and these teeth were undoubtedly vestigial. The 

 canines are so robust as to indicate that this was a 

 male animal. The cingulum is continuous around 

 the anterior face; the crowns are broken off. Close 

 behind the canines are the bifanged, well-developed 

 first premolars; these teeth are distinguished as in 

 M. giganteus by very broad, crenulate internal cingula 

 and prominent external cingula; in p^ and p^ the 

 tetartocones are very prominent. P* exhibits an 

 interesting example of correlated bilateral variation 

 in the double conical summits of the tetartocones; 

 this, however, is probably due to the advanced con- 

 dition of the cingulum. The hypocone is strongly 

 developed and entirely distinct from the cingulum 

 upon m^, as in Marsh's type Diconodon montanus and 

 in several specimens of M. trigonoceras. 



Relationship to Menodus. — In describing this sup- 

 posed genus, Menops, Marsh observed: "The present 

 genus is most nearly related to Diconodon and in its 

 molar teeth agrees with that form. It differs in the 

 presence of two upper incisors on each side." The 

 entirely vestigial character of the incisor teeth natu- 

 rally forbids our assigning them generic value. This 

 animal presents so many points of close similarity 

 with the type of M. ingens that the single decisive 

 specific character which can be selected is the abbre- 

 viation of the nasals and the somewhat less extreme 

 dolichocephaly. The nasals are less quadrate than in 

 M. ingens, the free length (105 mm.) being less than 

 the free width (140 mm.). Even this character is 

 possibly attributable to individual variation, since 

 some of the typical specimens of M. giganteus and M. 

 trigonoceras show a relative abbreviation of the nasals. 

 A similar broadening and abbreviation of the nasals 

 occurs in the subgenus Allops. The inferior contour 

 of the horns is less straight, and the horns apparently 

 diverge more widely than in M. ingens, a condition 

 partly attributable to the downward crushing of the 

 skull or perhaps indicating affinity with the fiat, 

 outwardly directed horns of Allops. 



The reduplication of the tetartocone on p'* is another 

 feature observed in Allops serotinus. A more impor- 

 tant difference is that the skull is proportionately 



shorter and less dolichocephalic than in M. ingens^ 

 the proportions being, length 760 millimeters, breadth 

 545, as compared with length 825, breadth 550. The 

 hypocone of ni'' is even larger and more distinct than 

 in most specimens of M. giganteus, resembling that 

 in the type of Diconodon montanus. This may be a 

 specific or progressive character. 



We observe the similarity to M. giganteus in many 

 other details, such as the sections of the hdrns, the 

 shape of the zygomatic arches, the presence of small 

 alveoli for the incisors, the antorbital knob on the 

 molars, the comparative length of the face, the ap- 

 parent exposure of the mastoid bone, the sharp and 

 horizontal shelf of the top of the occipital pillars, 

 the bifanged, well-developed first premolars, the ex- 

 treme cingulate development of the canines and grind- 

 ing teeth, the pi-ominence of the tetartocones and of a 

 hypocone on m^ 



Our conclusion is that the genus Menops is probably 

 equivalent to Menodus, while the species M. varians 

 may be regarded provisionally as valid and distinct. 



Menodus montanus (Marsh) 



{Anisacodon montanus Marsh, lS75; Diconodon montanus 

 Marsh, 1876; " Symborodon montanus" Osborn, 1902) 



Text figures 175, 447 

 [For original description and type references see p. 217] 



Type locality and geologic horizon. — Recorded as 

 from "northern Nebraska"; Titanotherium zone. 



Specific characters. — Nasals narrow, relatively ab- 

 breviate, so far as preserved, resembling those of 

 Allops serotinus, grinding teeth with transverse diam- 

 eter exceeding the anteroposterior, thus resembling 

 Allops. Incisors reduced or vestigial, as in Menodus, 

 m^ with a distinct hypocone, prominent external 

 cingula on all grinding teeth, and a strong internal 

 cingulum on the premolars. M'-m^ 218 millimeters. 



General characters. — The genotype (Yale Mus. 

 10022) of Anisacodon montanus is an incomplete 

 fragment of a skull, together with the complete 

 molar series of both sides, portions of the left 

 maxillary, of the left zygomatic arch, the ex- 

 tremities of the nasals, and the left superior pre- 

 molars. There are paired alveoli for p^ As in 

 Menodus the incisive border is narrow, sharp, and 

 functionally edentulous, but it exhibits two vestigial 

 alveoli. The premaxillary contains two caniae al- 

 veoli. The internal cingulum of the premolars is 

 prominent and rounded; the external faces of both 

 premolars and molars exhibit a sharply serrate cin- 

 gulum. The third superior molar presents a conical 

 hypocone quite distinct from the cingulum. To this 

 the generic name Diconodon, originally applied by 

 Marsh, refers. The grinding teeth are of about the 

 same size as those of Menops varians, the molar 

 fossettes are very deep. 



