EVOLUTION OF THE SKULL AND DENTITION OF OLIGOCENE TITANOTHERES 



525 



between the horns; the horn m its sub trihedral shape 

 and the position of its long axis is similar to that in 

 Menodus giganteus and is quite distinct from that of 

 the early members of either the Brontops or the Bronto- 

 therium phylum. Similarly the zygomatic section is 

 relatively deep and narrow, agreeing again with that 

 of M. trigonoceras. In consonance with dolichoce- 

 phaly, the postglenoid, post-tympanic, and paroc- 

 cipital processes are relatively short and simple. We 

 observe a well-defined postmastoid foramen, as in 

 other titanotheres. The posterior nares open between 

 the opposite m'. The basisphenoid is smooth. Un- 

 fortunately the teeth are so fractured that they yield 

 only one important fact — namely, that this is a very 

 old individual, and that its simple characters are not 

 juvenile but specific. 



Measurements of two specimens of Menodus heloceras, in 

 millimeters 



Basilar length 



Breadth, zygomata 



Height, temporal fossa. 

 Width, parietal plane.. 



Free length, nasals 



Free width, nasals 



Am. Mus. 6360 



(type) 



(Colorado) 



395 

 105- 

 104 

 100 

 100 



Am.Mus.l45"6 

 (Wyoming) 



603 



450 

 110 

 114 

 132 

 111 



<• Estimated. 



Slcull referred to Menodus Jieloeeras, from the hase of 

 the Titanotherium zone, Wyoming. — The type skull is 

 so poorly preserved that its measurements yield little 

 of interest. A skull which is provisionally referred 

 to this species (Am. Mus. 14576, fig. 436) comes from 

 the very base of the Titanotherium zone at Beaver 

 Divide, near Hailey, Wyo. It exhibits the following 

 comparative measurements: 



Measurements of Menodus heloceras, Alloys walcotti, and 

 Brontops brachycephalus, in millimeters 



Hence the skull referred to M. heloceras is distin- 

 guished by small molars, long nasals, and short horns, 

 which are all primitive characters. The form of the 

 horns and nasals suggests ancestral relationship to 

 Menodus or Allops. 



The upper molars, though poorly preserved, appear 

 to resemble those of Brontops hrachycephalus and differ 

 from those of Menodus heloceras in bekig relatively 

 wider. 



Menodus torvus (Cope) 



(Symhorodon torvus Cope, 1873; not "Symborodon torvus Cope," 

 Osborn, 1902) 



Plates CXXV, CXXVII, CXXXII, CLVIII, CLIX; text figures 



166, 396, 4.37 



[For original description and type reierences see p. 210] 



Type locality and geologic horizon. — Northeastern 

 Colorado, Titanotherium zone. 



Specific characters. — Lower jaw of Menodus type, 

 slightly -smaller than that of M. proutii; pi-ms, 310 

 millimeters; premolars much crowded (pi-p4, 100 

 mm.); incisive border apparently edentulous; an ex- 

 ternal cingulum on the canines, premolars, and molars; 

 Pi very small and closely crowded between the canine 

 and P2. 



The jaw (Am. Mus. 6365), chiefly described by 

 Cope, is the lectotype of the species "Symhorodon" 

 torvus, and this species is the genotype of his genus 

 Symborodon. This jaw proves to belong to the Meno- 

 dus phylum and to be unrelated to the "Symborodon" 

 of previous descriptions. 



The measurements of Am. Mus. 6365 given by 

 Cope are more or less inaccurate but serve to identify 

 the type. The following may be substituted: 



Measurements of Menodus torvus and M. trigonoceras, in 

 millimeters 



« Associated by Cope with type skull of S. bucco. ' Estimated. 



Measurements of premolars and molars in Menodus torvus {type) , 

 in millimeters 



Type lower jaw of M. torvus. — The ramus ascends 

 with a gentle curvature to the incisive border, which 

 is absolutely devoid of any traces of teeth or alveoli, 

 being thin and smooth with a small pit on its lower 

 surface (Pis. CXXV, B; CXXVII; fig. 437). The 

 incisor formula is therefore highly distinctive: !<>. 

 The canines were evidently small; the fangs measure 

 23 millimeters anteroposteriorly, 20 transversely; the 



