1883. | The Extinct Rodentia of North America. 169 
to the auricular meatus, which forms the handle to a hammer- 
shaped laminar bone. This is, no doubt, a dismemberment of the 
” squamosal, as a similar process is continuous with that bone in 
Thomomys, and one somewhat different is seen in Neotoma, 
Hesperomys, &c. Supraoccipital distinct on superior face of 
skull. Paroccipital process small or.none. Mastoid elongate, 
adherent to the otic tube. No postfrontal process. . 
A well-marked character which distinguishes the skull of this 
genus from Thomomys, Dipodomys, &c., is the separation of the 
meatal tube of the otic bulla from the zygomatic process of the 
Squamosal bone by an interspace. There is no postsquamosal 
d 
_Fic, 18.—Entoptychus planifrons Cope; skull, side, top, bottom, and posterior 
views. Nat. size. In fig. æ the roots of two molars are exposed, From John Day 
epoch, Oregon. Original. 
foramen in the recent genera. In Dipodomys the otic bulla is 
more largely developed, but it has the anterior bottle-neck pro- 
longation seen in Entoptychus. 
Individuals of this genus were very abundant in Oregon during 
the middle Miocene epoch. They represent several species, but 
how many it is difficult to determine. The most noteworthy 
Variations are found in the development of superciliary ridges; 
then there are modifications in the forms of the premolar teeth, 
differences in the length and width of the muzzle, and some range 
in dimensions. In Æ. crassiramis Cope (Fig. 17), there is a deep- 
ntal groove which is closed posteriorly. In Z cavifrons there 
are strong and in Æ Jambdoideus there are weak, superciliary 
