Cope.] "I" [Dec 7, 



These species differ as follows : 



I. Posterior ciugulum of true molars i and ii, wide, rising into a small 

 cusp. 



Length of true molars, M. . 0155 crassicollidens. 



II. Posterior cingulum distinct, thickened inwards. 



Length of true molars (m. ii inferential) .0175, crowns narrowed, trans- 

 verse bucculentus. 



Length of true molars .0170 ; crowns quadrate subtrigonus.* 



Length of true molars .0135 ; crowns narrowed, transverse 



{Chriacus) simplex. 



III. Posterior cingulum weak, disappearing inwards. 



Length of true molars .0105, crowns transverse except the third, which is 



very small '. incequidens. 



Cliar. Specif. The Tricentes crassicollidens is about the size of the 

 Chriacus truncatus and resembles it a good deal. The latter has, however, 

 a more transverse form of true molars, as compared with the present spe- 

 cies, where the form is subquadrate. In the present animal the premolars 

 are smaller, and if the third (second present) has an internal cusp, it is 

 much more insignificant than in the C. truncatus. These two species and 

 the Mioclmmis opisthacus resemble each other in the similar size, and in the 

 true molars having the posterior inner cusp more distinct than in other spe- 

 cies. They differ in the dimensions of their premolars, those of the M. 

 opisthacus being the largest, and those of G. truncatus being intermediate 

 in size. In the T. crassicollidens the anterior cingulum is also distinct. The 

 external cusps are conic, and are well separated, and the internal V is dis- 

 tinct. The internal cusp of the fourth premolar is small and compressed, 

 so as to be transverse. The base of the third premolar is triangular and 

 much longer than wide. All the superior molars, except the first premo- 

 lar, are furnished with an external cingulum, which rises into a more or 

 less distinct apex at its anterior and posterior angles. The first premolar 

 is a simple cone. The alveolus of the canine tooth is of large size. The 

 last true molar is not much reduced, and the first is as large as the second. 

 This is not the case with the T. bucculentus, where the first is considerably 

 smaller than the second. 



Measurements. M. 



Length of dental series to canine, exclusive 036 



" "diastema 006 



" " premolar series .0143 



*' " true molar series 0152 



Diameter of P-m. iv { anteroposterior 0042 



t transverse 0042 



* There may be two species confounded under this name. A specimen figured 

 in Vol. Ill of the final (4to) Report of the Hayden Survey, Plate XXIV, f, fig. 4 

 has four inferior premolars, all simple. 



