1896.] NATURAL SCIENCES OF PHILADELPHIA. 541 



as it exists in life and with the skin coloration and the distribution 

 of the pelage.'^* 



The skull of the Milrail animal is from H to 2 mm. larger in its 

 exterior dimensions than that of the type o^ glaber. It belongs to a 

 much older animal, and on this account the differences in dimen- 

 sions and formation of the teeth are, perhaps, largely attributable. 

 Among these the most noticeable are found, 1st, in the upper incis- 

 ors each bearing upon their inner anterior surftices a distinct shal- 

 low sulcus, bordered on the inner side by a sharp ridge and merging 

 outwardly into the convexity of the lateral two-thirds of the face of 

 tooth. Riippell states clearly that his animal had unchanneled in- 

 cisors ; Thomas says the incisors of phillipsi are " somewhat flattened 

 and bevelled on their interior halves," but does not define a sulcus. 

 The upper molars of the specimen in the Academy of Natural Sciences 

 of Philadelphia number six, as in glaber. Unlike those figured fur 

 glaher their crown surfaces are of unequal dimensions, m- 2 being 

 one-third larger than m- 1 and m- 3 considerably smaller than m- ^ . 

 In the two first upper molars the crowns have worn down until the 

 enamel folds are obliterated. In the last, which evidently erupted 

 at a much later date than the anterior pair, the crown shows a tri- 

 foliate surface, due to the impinging of the enamel walls of the lat- 

 eral and posterior sides of the tooth nearly to its center. Of the 

 three mandibular molars, m72 and m. 3 are about equal in size, m. 1 

 being about half as large ; the latter is circular in outline and shows 

 no enamel folding ; in m7~2 there is a pretty deep indentation on the 

 outer wall and a shallow curve of the inner ; in m. 3 these indenta- 

 tions are exaggerated, nearly equal, and nearly divide the tooth into 

 two sections, the anterior section being rectangular, the posterior 

 hemispherical in outline. If we were to apply the standard of specific 

 separation generally recognized to-day as governing the classifica- 

 tion of rodents, it would be consistent, perhaps, to make tlie third 

 specimen of Heterocephalus a third species on the dental characters 

 above defined, and on similar grounds establish a new genus for the 

 light-molared H. phillipsi, but I fully agree with Mr. Thomas 

 that the known individual variations in other species of the Bath- 

 yergincd are quite as marked as any yet attributed to Heterocepha- 

 lus. 



^^ A plate of the specimen is being prepared for Dr. Smith's book on the 

 Expedition. 



