MAMMALIA, 



e. Dentition hypsodont, i.e., molar teeth with vertically 

 lengthened crowns. . . EupetaufUS, p. 4°. ■ 



e^. Dentition brachyodont, i.e., molar teeth with short 

 crowns. 



/. An interfemaral membrane present ; tail generally 

 bushy. . . . Pteromys, p. 32- 



/^. No interfemoral membrane ; tail distichous. 



Sciuropterus, p. 37- 



d^. Limbs free ; form agile ; tail long, bushy ; no cheek 

 pouches; parietals broad, anchylosed to the frontals. 



Sciurus, p. 4. 



c^. Incisors not compressed ; form thick-set ; tail short ; pollex 

 rudimentary ; parietals narrow, parallelogrammatic. 



Arctomys, p.4i. 



'. Premolars present or absent ; no post-orbital processes ; 

 zygomatic arch slender, chiefly formed by the zygomatic 

 processes of the maxillae and squamosal bones ; mandible 

 with the angular portion springing from the lower edge pf 

 the bony covering of the lower incisor; tibia and fibula 

 completely anchylosed in the adult; upper lip usually 

 cleft ; muffle small, naked ; nostrils comma-shaped ; tail 

 cylindrical. [=Myomorpha.] 



/". Lower incisors generally compressed ; no premolars; infra- 

 orbital opening generally high, wide above, narrow below, 

 with the maxillary process forming a perpendicular plate ; 

 tail generally naked. [=Muridae.] 



g. Molars rooted. 



A. Molars transversely laminated ; incisive foramen and 

 auditory bulla very small ; palate imperforate ; form 

 myoxine ; fur mixed with scattered spines ; tail 

 densely haired. . Platacanthomys, p. 46. 



A^. Incisors narrow, upper ones grooved ; molars trans- 

 versely laminated ; auditory bulla large ; hind limb 

 elongated ; tail long and hairy. Gerbillus, p. 46. 



h^. Incisors not grooved ; molars generally tuberculate ; 

 tail scaly, and generally more or less naked. 



J. Only the pollex provided with a nail, all Other digits 

 clawed. 



k. Lower incisors broad ; molars of transverse 



