1881.] 15'' [Cope. 



of the Big-Horn, and a good many of them do not coincide well in 

 characters with the species already described. I define them as follows, 

 premising that with other parts of the skeleton some changes may be found 

 to be necessary. The large D. altidens was not obtained by Mr. Wortman 

 in the Big-Horn country. 



I. Inferior tubercular molar oval in outline, with a heel. 



Length true molars .010 ; last three premolars .0135 ; last molar narrow. . 



D. dawkinsianus. 

 Length true molars .016 — .018 ; last three premolars .038 — .030 ; last 



molar narrow D. leptomylus. 



Length true molars .019 — .020; last three premolars .036; last molar 



elongate D. protenus. 



Length true molars .035 ; last three premolars .035 ; last molar short 



D. altidens. 



II. Inferior tubercular molar short, subquadrate in outline. 



Length true molars .011 ; depth of ramus at sectorial .010 



D. masHetericus. 

 Length true molars .018 ; depth of ramus at sectorial .017 D. curtidens. 



DiDYMICTIS LEPTOMYLUS CopC. 



American Naturalist, 1880, p. 908. 



The specimens which I refer at present to this species belong to two 

 varieties, which may perhaps be specifically distinct ; but this cannot be 

 demonstrated at present. They differ in dimensions only. Thus the true 

 molars of the type, which comes from the Big-Horn beds, measure 

 M. .016 in length. Five specimens from the Big-Horn basin agree in hav- 

 ing this dimension .018. The entire inferior molar series is only a little 

 shorter than that of the smaller variety of the Z). protenus from New 

 Mexico (See my report to Capt. Wheeler, plate xxxix). 



DlDYMICTUS PKOTENUS CopC. 



Jaws more or less complete, of six individuals, are referable to this 

 species. They agree closely in measurements and belong to the larger 

 variety of the species figured on plate xxxix of the report to Capt. 

 Wheeler. 



DiDYMICTIS MASSETEKICUS, Sp. nOV. 



This species is intermediate in size between the D. Uptomylus and the 

 D. daiokinsianus, and is characterized by the peculiar form of its tubercular 

 molar, and the deeply excavated masseteric fossa. It appears to have been 

 a rare species, as only one mandibular ramus was found by Mr. Wortman. 

 This is brolven off in front of the fourth premolar, and supports the last true 

 molar teeth. 



The tubercular molar is subquadrate in form, and consists of three low 

 tubercles in front, and a wide heel behind, which has an elevated posterior 

 border. The tubercular-sectorial has a short and narrow heel. Its anterior 

 cusps are not very acute, and the two internal are equal, and a good deal 



