Cope.] -"* [Nov. 15. 



and 8. pansus. The posterior fosscttes of the crowns seen in those species 

 arc wanting in the S. gradatus. 



Extoi- ivciii s cavifbons, gen. et sp. nov. 



Char. gen,. Probably of the family Saccomyida.* The cranium is 

 elongate, and presents inflated periotic bones, and slender zygoma. The 



foramen infraorbital! 1 is small and anterior in position, entering the maxil- 

 lary hone near its suture with the premaxillarv. 



Generic characters. Molars j \, rootless, and identical in structure. The 

 crowns arc prismatic, and in the young stage present a deep inflection 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 transverse fossette, entirely 

 enclosed 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 differ from those of Perognatkus in being with- 

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

 Dipodomys, the molars are undivided simple prisms. 



Specif. Char. This species is represented by some entire crania, and 

 numerous separated jaws. The postorbital part of the skull is subquadrate 

 in outline, and depressed in form. The interorbital region is narrowed, 

 but the superciliary margins d'j not meet nor converge to form a sagittal 

 crest. They are thickened, forming two subparallel ridges which are sepa- 

 rated by a shallow concavity of the frontal hone. The nasal hones are very 

 narrow, and their posterior apices just attain the line of the supero-anterior 

 angle of the orbit. The base of the malar hone is much elevated and very 

 oblique. The molar teeth are directed obliquely backwards, the alveolus 

 of the first issuing below the anterior part of the orbit. The first superior 

 molar is the largest, and the proportions of the others diminish regularly 

 posteriorly. The first inferior molar is a little smaller than the second and 

 third, and is about equal to the fourth. Its anterior column is contracted, 

 while the last molar is like the second and third. The face of the inferior 

 incisor is flat, and its enamel is smooth. The external face of the jaw is 

 bounded below by a strong angle, as far anteriorly as below the fust molar. 



Measurement'}. M 



Length of skull to incisive alveoli .041 



Width of skull at mastoids 020 



• ' between orbits 005 



" "at middle of muzzle 010 



Elevation of skull from second molar 011 



Length of molar series 007 



" first molar 002 



Widthof " " ....002 



Length of crown of last molar 0015 



*See Cones- Report U. S. Geol. Snrv. Terrs. XI, ]». 101. 



