U. S. GEOLOGICAL SURVEY 



MONOGRAPH 55 PLATE CXXV 



JAWS OF MENODUS PROUTII AND MENODUS TORVUS 



, Menodus proutii, type (Nat. Mus. 113) (genotype of Tit another ium Leidy, see pp. 205, 527), "Mauvais Terres * * * 

 of White River," Chadron formation. One-third natural size Generic agreement with the genotype of Menodus 

 giganteus Pomel (see p. 204) is seen in the elongate molars with sharp external cingula. The jaw fragment and teeth 

 also agree in generic characters with the jaws Am. Mus. 1007, 1067 (see PI. CXXXIV, A'), referred to Menodus 

 Menodus proutii, as represented by this jaw and by the skull Am. Mus. 9335, has the dimensions of 

 as and is much sm,aller than M. giganteus. A-, M. proutii, imperfedl third low^er molar of type (Am. 

 evi/. (See A'.) One-half natural si^e. The tooth shows w^ell the heavy external cingulum 

 Menodus torvus, type of Symhorodon torvus Cope (Am. Mus. 6365), Horsetail Creek, north- 

 ea^em Colorado, Chadron form.ation. One-fourth natural size. This is the true genotype of Symborodon, because 

 Cope in his original description of Symborodon torvus (see p. 211) based the generic description primarily upon this 

 jaw. It shows generic affinity with Menodus giganteus, and hence Symhorodon becomes a synonym of Menodus. 

 The incisors were wanting or ve^igial, the premolars and molars were somewhat elongate v/ith heavy external 

 cingula, and the general form, of the ja-w recalls that of Menodus. The front view (B^) shows the edentulous i 

 border. In size this type of M. torvus is intermediate between M. heloceras and M. proutii 



trigonoceras 

 M. trigonoci 

 Mus. 113) 

 of Menodi 



