SCIUROMOKPHA. 845 



anterior to the external inflection. The latter has become isolated from the 

 superficial enamel on the last three molars, by attrition. The superior 

 incisors are flat anteriorly with the external angle rounded, and its dentine 

 presents the transverse undulation seen in S. pamus. 



The palate is distinctly narrower in this species than in the C. penin- 

 sulatus. The width between the bases of the first molars (Pm. iv) is les* 

 than the transverse diameter of each of those teeth ; in C. penhisulatm it* 

 width is equal to that of those teeth. The temporal ridges do not unite so- 

 early in this species as in C. peninsulatus, but continue parallel for a consid- 

 erable distance. The parietal bone is injured posteriorly from the middle, 

 so that the union into a sagittal crest, if it takes place, cannot be seen. The 

 middle line of the basioccipital bone is keeled, with a fossa on each side. 

 The tympanic meatus is prolonged, and the posttympanic process is short. 

 The space between the incisor and superior molar teeth is about two-third& 

 the same space in the C. peninsulatus, and is relatively shorter than in that 



species. 



Measurements. 



Ji. 



Length of skull from incisive alveolus 0500 



Widti between bases of first molars 0030 



Width between bases of fourth molars 007O- 



Length of molar series 0115 



Diameter ofthe first molar ^'^"t^"""!*"'**''"*'^ OO"" 



( transverse 004&- 



0028 



0032 



Diameter of fourth molar 5 '^^^^"P"^'^"'"" 002t>- 



( transverse 0024 



Diameter of third molar ^ anteroposterior 

 c transverse 



From the above measurements it is apparent that the molar series in. 

 this species is equal in length to the anterior three molars of the S. nehras- 

 censis and S. pansiis. The posterior fossettes of the crowns seen in those- 

 species are wanting in the S. gradatus. 



From the John Day Rivev, Oregon. 



HELISCOMYS Cope. 



Synopsisof New Vertebrata from Colorado (Miss. Pub., U. S. Geol. Survey Terrs.), 1P73 (October), p.Sj. 

 Annual Report U. S. Geol. Survey Terrs., 1873 (1874), p. 475. 



Inferior molars four, rooted, the crowns supporting four cusps in trans- 

 verse pairs A broad ledge or cingulum projecting on the external side^ 



