482 



Anterior portion of zygomatic arch formed chiefly 



by the jugal bone ; infraorbital foramen small. 



Skull without postorbital processes ; molars 



rootless ; tail broad and Hat (Beavers) Castoridx, p. 9-47. 



Skull with postocbial processes ; molars rooted ; 

 tail terete. 

 Fore and hind leg not joined by a fold of 



skin (Squirrels) Sciurichv,-p. 897. 



Fore and hind leg joined by a fold of skin 



acting as a parachute (Flying Squirrels) . Petaurisiidn-, p. 940. 

 Anterior portion of zygomatic arch not formed 

 chiefly by the jugal bone ; infraorbital fora,- 

 men large. 

 Jugal bone not supported by long zygomatic 

 process of maxillary : mandible with 

 angular part arising from outer side of 

 alveolus of incisor ; tibia and fibula dis- 

 tinct ; body covered with long quills 

 (nasal and frontal regions of skull in 

 European genus conspicuously inflated) 



(Porcupines) HystricicLv, p. 542. 



Jugal bone supported by long zygomatic pro- 

 cess of maxillary ; mandible with angular 

 part arising from under side of alveolus 

 of incisor ; tibia and fibula joined ; body 

 not covered with quills. 

 Lachrymal and malar bones in contact ; ante- 

 orbital foranaen much wider below than 

 above, its inner border with conspicuous, 



nearly closed secondary canal Zapodidx, p. 535. 



Lachrymal and malar bones not in contact ; 

 anteorbital foramen not wider below than 

 above, its inner border without secondary 

 canal. 

 External form excessively modified for 

 underground life ; skull wedge-shaped, 

 the supraoccipital sloping forward to 

 middle of brain-case, its area much 

 greater than that of parietals (Mole- 

 rats) SpalacidcV, p. 887. 



External form not specially modified for 

 underground life ; skull not wedge- 

 shaped, the supraoccipital vertical or 

 nearly so, confined to ba'ck of brain-case, 

 its area much less than that of parietals. 

 Cheek-teeth (in European genera) | ; 



cfecum absent (Dormice) Muscardinidse, p. 549. 



Cheek-teeth (in European genera) § ; 



caecum present Muridie, p. 591. 



Molars prismatic, hypsodont, usually 

 rootless, their crowns flat (Voles 



and Lemmings) MicroUnie, p. GIO. 



Molars tuberculate, brachyodont, 

 rooted. 

 Tubercles on crowns of upper molars 

 arranged in two longitudinal 



series Cn'cetinoe, p. 592. 



Tubercles on crowns of upper molars 

 arranged in three longitudinal 

 series Murinx, p. 791. 



