856 THE JOHN DAY FAUNA. 



The teeth of this genus differ from those of Perognathus in being with- 

 out distinct roots, and in having the enamel loop cut off and inclosed. In 

 Dipodomys, the molars are undivided simple prisms. 



The skull is compact, and does not display the vacuities or large fora- 

 mina seen in some genera oi Rodentia. The incisive foramina are rather small 

 and posterior in position. There is a foramen on the side of the alisphenoid, 

 which is nearly in the position of the anterior alisphenoid canal of the 

 TJiomomijs hidhivorus. The foramen rotundum is immediately below and 

 within the anterior part of the glenoid cavity. The foramen ovale is not 

 distinct from the foramen lacerum anterius, and is on the external side of 

 the apex of the petrous bone. The other foramina lacera are closed, so that 

 the carotid foramen pierces the inner side of the otic bullae. The condy- 

 loid foramen is close to the occipital condyle. The meatus auditorius 

 externus is at the extremity of a tubular elongation of the bulla, and is 

 separated by a space from the zygomatic process of the squamosal bone. 

 Between the bases of these is a fossa which is bounded above by a ridge 

 as in the genus Castor. Below this ridge is a subsquamosal foramen, and 

 above it a postsquamosal. There are no 2:>ostparietals nor mastoid foramina. 



There are deep pterygoid fossae, whose inner bounding laminae unite 

 on the middle of the palatine border, and whose external laminae are con- 

 tinuous with the posterior extremity of the maxillary bone. The otic bullae 

 are not separated very distinctly from the mastoid. The latter looks like a 

 continuation of the former, as in Thomomi/s, and occupies considerable 

 space between the exoccipital and the squamosal. The latter sends down- 

 wards a process just posterior to the auricular meatus, which forms the 

 handle to a hammer-shaped laminar bone. This is, no doubt, a dismem- 

 berment of the squamosal, as a similar process is continuous with that Ijone 

 in Thomomps, and one somewhat different is seen in Neofoma, Hesj^erowi/s, 

 &c. Supraoccipital distinct on superior face of skull Paroccipital process 

 small or none. Mastoid elongate, adherent to otic tube. No postfrontal 

 process. 



A well-marked character which distin_guislies the skull of this genus 

 from TJiomomys, D'qjodotnys, &c., is the separation of the meatal tube of the 

 otic bulla from the zygomatic process of the squamosal bone by an inter- 



