436 MILLER AND GIDLEY : SUPERGENERIC GROUPS OF RODENTS 



Family Muscardinidae 



Skull with no striking modifications of general form; zygomatic root, 

 much as in the Sciuridae except that its anterior face is nearly vertical 

 instead of strongly oblique, and the infraorbital foramen extends above 

 median level of orbit, receiving or transmitting a strand of muscle as 

 well as the nerve; no postorbital processes; auditory bullae large, globu- 

 lar, rounded in front; cheekteeth ^, brachydont (in Leithia subhypso- 

 dont), the enamel pattern reduced-hexamerous in forms with basin- 

 shaped crowns, passing to a system of parellel transverse ridges in 

 those with flat crowns (parallel : Graphiuridae) ; external form showing 

 a combination of murine and sciurine features. 



Eliomys, Dyromys, Glis, Muscardmus, Leithia; Old World Middle 

 Miocene to Recent. 



FOUR-CUSPED SERIES 



Modifications of teeth based on an underlying quadritubercular 

 structure. 



Family Ischyromyidae 



General characters of the skull as in the Muscardinidae; teeth f, 

 moderately hypsodont, rooted, the fundamental structure quadri- 

 tubercular, the enamel pattern in worn teeth reduced-heptamerous. 



Ischyromys; North American Middle Oligocene. 



Family Cricetidae 



Fundamental zygomasseteric structure as in the Muscardinidae and 

 Ischyromyidae, but infraorbital foramen usually enlarged and special- 

 ized, consisting of a rounded upper portion for transmission of muscle 

 and a narrow lower portion for transmission of nerve, the zygomatic 

 root developed into a broad, oblique plate; skull varying excessively in 

 form, but always without postorbital process on the frontal; check- 

 teeth f , the crown structure showing all stages from brachydont to 

 ever-growing, the fundamental structure quadritubercular, the enamel 

 pattern varying from simple heptamerism to excessive specialization, the 

 tubercles in the maxillary teeth always presenting a longitudinally bi- 

 serial arrangement and never developing a functional third series on 

 lingual side of crown; external form murine or fossorial. 



Subfamily Cricetinae. — Skull without special modification, the zygo- 

 masseteric structure as usual in the family, the squamosal not devel- 

 oping a postorbital ridge or process; molars rooted, their crowns vary- 

 ing gradually from tubercular and brachydont to flat-crowned and 

 strongly hypsodont, when in the latter condition the prisms not oppo- 

 site (compare Gerhillinae) and the posterior termination of m^ and m- 

 not angular (compare Microtinae). 



The Cricetinae, Sigmodontinae, Neotominae, and Nesomyinae of authors; 

 Oligocene to Recent; continental region of the world; Madagascar. 



