442 MILLER AND GIDLEY." SUPERGENERIC GROUPS OF RODENTS 



Family Idiuridae 



Like the Anomaluridae but skull with infraorbital foramen greatly 

 enlarged, the lower zygomatic root nearer to incisor than to anterior 

 cheektooth; anterior point of masseteric insertion on mandible in front of 

 pm''; incisors excessively heavy; cheekteeth weak, extremely brachydont, 

 their crowns flat, wider than long. 



Subfamily Idiurinae. — Flying membrane present; cheekteeth with 

 two complete m\3dian transverse ridges. 

 Idiurus; Recent; Africa. 



Subfamily Zenkerellinae — Flying-membrane absent; cheekteeth with 

 one complete median transverse ridge. 

 Zenker ella; Recent; Africa. 



D, — Skull without striking peculiarities other than a tendency to assume 

 a form characterized by broad braincase, large auditory parts, and weak 

 rostrum {parallels; Gerbillinae, Octodontinae) ; infraorbital foramen trans- 

 mitting muscle in all members of the group in which the skull is known 

 except probably Sciuravus ; cheekteeth varying from brachydont to ever- 

 growing, their modifications based on a heptamerous structure in which the 

 ridges are wide and the reentrant spaces narrow. 



Family Sciuravidae 



Infraorbital foramen small, but visible in lateral view of skull, prob- 

 ably transmitting nerve only; cheekteeth f, brachydont; the structure 

 of the upper molars obviously and simply quadritubercular. 



Sciuravus; Noi'th American Middle Eocene. 



Family Zapodidae 



Infraorbital foramen large, transmitting muscle as well as nerve; 

 cheekteeth varying in number from f in the earlier members of the group 

 to f in the most advanced; the quadritubercular crown structure usually 

 though not always much modified; metatarsals not reduced or fused. 



Subfamily Theridomyinae. — The earlier, less modified members of the 

 family: pm* a large, functional tooth; crowns of cheekteeth varying 

 from brachydont and simply quadritubercular (Sciuroides) to hypsodont 

 and much reduced heptamerous [Issiodoromys; parallel: Eocardia). 



The Theridomyidae of authors; European Lower Eocene to Miocene. 



Subfamily Sicistinae. — Cheekteeth brachydont, |, distinctly quadri- 

 tuberculate, the enamel of moderately worn upper molars with a simple 

 heptamerous pattern ; external form murine, the hind legs and feet not 

 lengthened. 



Sicista, Recent, Eurasia; f Eomys, European Upper Eocene. 



