378 BULLETIN UNITED STATES GEOLOGICAL SURVEY. [YoUV.- 



ENTOPTYCHTJS Cope. 



Paleontological Bulletin No. 30, p. 2, December 3, 1878; Proceedings American Philo- 

 sophical Society, 1878 and 1879, p. 64. 



Family Saccomyidce.* The cranium is elongate, and presents inflated 

 periotic bones, and slender zygoma. The foramen infraorbitale is small 

 and anterior in position, entering the maxillary bone near Its suture 

 with the premaxillary. 



Generic characters. — Molars |— f , rootless, and identical in structure. 

 The crowns are prismatic, and in the young stage present a deep inflec- 

 tion of enamel from one side, the external in the superior teeth, the 

 internal in the inferior. After a little attrition, the connection with the 

 external enamel layer disappears, and there remains a median trans 

 verse fossette, entirely inclosed by enamel. The tooth then consists of two 

 dentinal columns in one cylinder of enamel, separated by a transverse 

 enamel- bordered tube. Incisors not sulcate. 



The teeth of this genus difterfrom those of PerognatMis in being with 

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

 In Dipodomys, the molars are undivided simple i)risms. 



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

 mina seen in some genera of Bodentia. 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 Thomomys hulbii'orus. The foramen rotundum is imme- 

 diately 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 ex ernal side of the apex of the petrous bone. The other fora- 

 mina lacera are closed, so that the carotid foramen pierces the inner 

 side of the otic bullae. The condyloid foramen is close to the occipital 

 condyle. The meatus anditorius externus is at the extremity of a tubu- 

 lar elongation of the bulla, and is separated by a space from the zygo- 

 matic 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 i)OStsquamosal. 

 There are no postpareitals nor mastoid foramina. 



There are deep pteiygoid fossae, whose inner bounding laminae unite 

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

 continuous 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 Thomomys, and occu- 

 pies considerable space between the exoccipital and the squamosal. 

 The latter sends downwards a process just ijosterior 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 pro- 

 cess is continuous with that bone in Thomomys, and one somewhat dif- 

 * Geomyidoi Alston. 



