EVOLUTION OF THE SKULL AND DENTITION OF OLIGOCENE TITANOTHERES 



527 



Characters of the type (Jedotype) lower jaw. — The 

 reasons for regarding this lower jaw fragment (Nat. 

 Mus. 113) as the type, or lectotype, of this species 

 are given on page 210 of this monograph. The speci- 

 men consists of part of the left ramus of the lower 

 jaw containing the posterior alveolus of ps, both 

 alveoli of p4, and the three molars, which are much 

 worn and somewhat damaged in certain places. The 

 molars exhibit the strongly de- 

 veloped external cingula that are 

 characteristic of the true Menodus, 

 and the measurements show that 

 we have here a small member of 

 the Menodus phylum, distinctly 

 larger than the very primitive 

 M. heloceras but somewhat smaller 

 than the typical M. trigonoceras of 

 the middle Titanotherium zone. 



Measurements oj Menodus 

 proutii. — In the type lower jaw of 

 this species the true molar series 

 (234 mm.) is about 5 to 10 milli- 

 meters shorter than in jaws re- 

 ferred to M. trigonoceras. The 

 specific name proutii is accordingly 

 applied to the smaller members of 

 the "trigonoceras" group. If we 

 adopt the trinomial system of 

 nomenclature the specific name 

 proutii might include M. proutii 

 proutii for the smaller skulls, and 

 M. proutii trigonoceras for the 

 larger skulls. 



The molars of the type are 20 

 millimeters longer than in the type 

 of Symhorodon torvus Cope, which 

 is also a menodont, and the jaw as 

 a whole was considerably larger 

 than those referred to AUops 

 marshi. 



In a National Museum skull 

 (No. 4701, ? ) referred to M. 

 proutii the premolar-molar series 

 is 45 millimeters shorter than in 

 the typical M. trigonoceras, the basilar length is also 

 much shorter, and the zygomatic index (74, estimated) 

 is relatively high. 



Description of the neotype of M. proutii. — The upper 

 jaw selected by Osborn as a neotype is a specimen 

 (Am. Mus. 9335) from Lance Creek, Wyo., recorded 

 from the middle Titanotherium zone. 



The upper molars of the neotype are of conformable 

 size with the lower molars of the type. In fact, the 

 well-worn lower premolar-molar series of the type 

 jaw has been found to articulate fairly closely with 



the less worn upper premolar-molar series of the 

 neotype maxilla (fig. 438). The neotype upper grind- 

 ers exhibit the characteristic internal and external 

 cingula of Menodus, lofty ectolophs, tetartocones 

 strongly developed on p^, p^, and p*, a prominent tri- 

 angular hypocone on m', and molar crowns distinctly 

 elongated anteroposteriorly; they also exhibit rudi- 

 ments of the crochet and antecrochet. 



Figure 437. — Lower jaws of Menodus {Symhorodon) torvus and M. trigonoceras 



Menodus torvus, Am. Mus. 6366 (type) ; a very aged animal with angle very prominent and truncate, cheek teeth 

 with sharply defined external cingula, and incisive border edentulous as in Menodus. B, M. tngonoceras, Nat. 

 Mus. 4745; a very characteristic jaw, showing rather full symphyseal region, angle produced downward and 

 backward, canines and cheek teeth sharply cingulate and subhypsodont. One-fifth natural size. 



Additional measurements of Am. Mus. 9335 (neo- 

 type of M. proutii) are given below. 



Millimeters 

 P2, ap. bytr 26X28 



Millimeters 



Canine, vertical (esti- 

 mated) 39 



Canine, anteroposterior.- 24 



P'-m3 313 



pi-p4 119 



M'-m' 192 



P^ap. bytr 34X41 



F\ ap. by tr 38X49 



Ml, ap. by tr 59X04 



M2, ap. bytr 69X62 



M3, ap. bytr 69X67 



Dolichocephaly is strongly marked in m^~'. The 

 measurements show that this specimen is smaller 

 than Carnegie Mus. 3068. 



