846 THE WHITE EIVER FAUNA. 



from base of the cusps. The inferior incisor compressed, not grooved, and 

 with the enamel without sculpture. 



This genus is only represented by a small number of specimens, which 

 are mandibular rami exclusively. Its special affinities, therefore, cannot be 

 ascertained, and even its general position remains somewhat doubtful. 

 There is some probability, however, that it belongs to the Myomorpha, as 

 the type of dentition is much more like that of the genera of that group 

 than those of the Sciuromorpha. To the Hystricomorpha it does not belong. 



As compared with known genera of Myomotpha, it is at once separated 

 from many of them by the presence of a premolar tooth. Among recent 

 genera of this suborder, Sminthus possesses this tooth in both jaws, and 

 Meriones in the upper jaw only. It is present in both jaws in the Sciuro- 

 mopha generally. The tubercles of the teeth resemble those of the Muridm, 

 but their disposition is unlike that of any existing North American genus. 

 A remote approximation to it is seen in the genus SyllopJwdm of the Bridger 

 Eocene formation, where there are four subquadrate molars with tubercles; 

 but the latter form two transverse crests, with an additional small inter- 

 mediate tubercle, and the wide cingulum is absent. 



But one species of Heliscomys is known, the H. vetus. 



Heliscomys vetus Cope. 



Synopsis of New Vertebrata Colorado, 1873 (October), p. 3. Annual Report U. S. Geol. Survey Terra., 



1873(1874), p. 475. 



Plate, LXV: figs. 14-18. 



The mandibular ramus is rather robust in its proportions. Neither the 

 alveolar sheaths of the molars nor that of the incisor project beyond the 

 general surface. The anterior base of the ascending ramus commences 

 opposite the anterior part of the last molar tooth. The masseteric fossa 

 extends remarkably far anteriorly, its inferior border terminating close to 

 the mental foramen. This border is moderately raised, and extends down- 

 wards and backwards, not reaching the inferior border anterior to a point 

 below the anterior base of the ascending ramus. The symphysis rises at 

 an angle of about 45°, and the inferior border is very little convex to below 



