1883.] The Extinct Rodentia of North America. 167 
they have not been found in the Loup Fork formation, but 
they occur in the : 
Pliocene Equus 
beds. Two genera 
are known, the one 
above named, and 
Entoptychus Cope. 
They are very nearly 
allied to existing 
genera. In the 
former the molars € $ r 
are rooted and have ra. tiad, Piirit leptophrys Cope; skull, a 
Dnt aaan, and a or aet a ae ae a RE t 
fold of enamel on From the John Day epoch, Oregon. Original. From Volg 
oneside ofthecrown ™ Report U.S. Geol. Surv. Terrs. 
is always open. In the latter, the molars are prismatic and rootless, 
and the lateral enamel fold becomes on wearing an isolated lake. 
Pleurolicus is curiously near to the existing Heteromys and 
Perognathus, the two genera of Saccomyidæ 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, 
šo faras 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 £. 
Planifrons did not reveal any roots. It is also opposed to this 
view that the maxillary bone in the Pleurolici has little depth be- 
low the orbital fossa, appropriately to the short-rooted molars, 
while the depth is considerable in the typical Entoptychi, though 
is a complete gradation in this respect. But I have demon- 
ted satisfactorily that Pleurolicus is a distinct genus by obser- 
vations on the P. leptophrys. Some of my individuals of this spe- 
cles are young, with the crowns of the molars little worn, yet the 
Toots diverge immediately on entering the alveolus, on all the 
_ ‘Molars. In the species of Pleurolicus the lateral fissure of the 
