866 THE JOHN DAY FAUNA. 



PLEUROLICUS Cope. 



Paleontologioal Bulletin, No. 30, p. 3, Dec, 1878. Proceed. Amer. Philo. Soc.,.1878 (1879), p. 66. 



Family Saccomyidce. Superior molars rooted and short-crowned. The 

 crowns with a lateral fissure bordered with an inflection of the enamel 

 sheath, extending- to their bases. In the superior molars this inflection is 

 on the external side, and does not divide the crown. Superior incisors not 

 grooved. 



This genus is curiously near to the existing Heteromys and Perognathus, 

 the two genera of Saccomyidce with rooted molars. The former differs in 

 having: the molars divided into two columns, each of which is sheathed in 

 enamel, while Perognathus only differs, so far as I am aware, in having the 

 superior incisors grooved. It is also very nearly related to Entoptychus, 

 and two of the species correspond in various respects with two of those 

 of that genus. In view of the fact that most of the specimens of the P. 

 sulcifrons are old individuals with well-worn molars, the idea occurred to 

 me that the rooted character of the molars might be common to the species 

 of Entoptychus, but that it might not appear until long use had worn away 

 most of the crown, and the protrusion had ceased. Examination of the 

 bases of the long molars of E. planifrons did not reveal any roots. It is 

 also opposed to this view that the maxillary bone of the Pleurolici has little 

 depth below the orbital fossa, appropriately to the short-rooted molars, 

 while the depth is considerable in the typical Entoptychi, though there is a 

 complete gradation in this respect. But I have demonstrated satisfactorily 

 that PleuroUcus is a distinct genus by observations on the P. leptophrys. 

 Some of my individuals of this species are young, with the crowns of the 

 molars little worn; yet the roots diverge immediately on entering the alve- 

 olus on all the molars. In the species of PleuroUcus the lateral fissure of the 

 crown descends to its base, and hence persists longer than in the typical 

 Entoptychi. 



I am acquainted with two species of this genus. The posterior part 

 of the skull of an individual represents a third species, which I refer pro- 

 visionally to this genus. 



