1876.1 MR. E. R. ALSTON ON THE ORDER GLIRES. 77 



2. Sciurus, Linnaeus, Syst. Nat. i. p. 86 (1706). 



Limbs free, form agile, tail long, bushy. No cheek-pouches ; 

 tbree or four pairs of teats. First upper premolar sometimes soon 

 lost. Frontals ankylosed with parietals ; postorbital processes 

 moderate ; infraorbital opening in front of anterior root of zygoma. 

 Palate broad, flat. 



3. Xerus, Hemprich & Ehrenberg, Symbol. Phys., Mamm. i., gg 



(1832). 



Ears very short or rudimentary, tail short, fur sparse, harsh, 



with flattened spines. No cheek-pouches, two pairs of teats. Nasals 



and palate narrower, and postorbital processes much smaller than in 



Sciurus. 



4. Tamias, Illiger, Prod. Syst. Mamm. p. 83 (1811). 



Ears short, fore feet with the fourth digit longest, limbs subequal, 



tail short. Large internal cheek-pouches. First upper premolar 

 soon lost. Skull slender ; infraorbital opening in anterior root of 

 zygoma, not in front of it. 



B. Arctomyin.e. Incisors not compressed. Limbs free, form 

 usually stout, tail short. Palaearctic and Nearctic. Recent genera : — 



5. Spermojihilus, F. Cuvier, Mem. du Mus. vi. p. 293 . (1822). 



Form somewhat slender ; tail short or moderate. Claw of pollex 

 rudimentary or absent. Large cheek-pouches. Series of grinding- 

 teeth nearly parallel. Skull with no marked ridges ; postorbital 

 processes slender, directed backwards. 



6. Cynomys, Rafinesque, Amer. Monthly Mag. ii. p. 45 (1817). 

 Form thickset, tail short, claws of fore feet long on all the digits, 



shallow cheek-pouches. Series of grinding-teeth strongly convergent 

 behind. Skull short and broad ; postorbital processes long, directed 

 backwards ; parietals narrow, parallelogram made. 



7. Arctomys. Schreber, Siiugethiere, iv. p. 721 (1792). 



Form thickset, tail short ; pollex rudimentary, with a flat nail. 

 Cheek-pouches rudimentary or absent. Series of grinding-teeth 

 nearly parallel. Skull broad ; postorbital processes large, trian- 

 gular, standing out at right angles ; parietals narrow, parallelogram- 

 matic. 



Fossil genera. The following genera, characterized from details 

 of dentition, seem to be referable to this family : — Plesarctomys, 

 Bravard, in Gervais's ' Zool. et Pal. Franc.' pi. xlvi. (1852), Eocene 

 of France; Pseudosciurus, Hensel, Z. Deutsch. geol. Ges. 1856, 

 p. 660, bone-beds of Wurttemberg ; Sciuravus, Marsh, Am. Joum. 

 Sc. 1871, p. 120, Eocene of North America ; Paramys, Leidy, Geol. 

 Survey, Montana, 1871, p. 363 (perhaps the same as the last) ; Gym- 

 notrichns, Cope, Pal. Bulletin, i. p. 6 (1874), Miocene of North 

 America. 



