MILLER AND GIDLEY : SUPERGENERIC GROUPS OF RODENTS 433 



tilted upward, usually broad, with its superior border always above 

 lower margin of infraorbital foramen. Infraorbital foramen inferior, 

 transmittng nerve only; masseter lateralis passing obliquely upward to 

 superior border of rostrum, always to exclusion of masseter medialis. 



THREE-CUSPED SERIES 



Teeth becoming hypsodont on the basis of a tritubercular structure. 



Family Sciuridae 



Skull never truly fossorial; infraorbital foramen with outer wall 

 usually though not always forming a distinct canal, its orifice protected 

 from muscular action by the presence, at or near its lower border, of an 

 outgrowth for attachment of masseter lateralis superficialis; frontal 

 with decurved postorbital process; cheekteeth brachydont or uni- 

 laterally hypsodont, the fundamental tritubercular plan usually (prob- 

 ably always) evident in functional adult teeth that have not under- 

 gone considerable wear; external form suited to arboreal or terrestrial 

 life. 



The Sciuridae of authors. 



Subfamily Sciurinae. — Orbital region normal, the middle of orbit in 

 front of middle of skull (except in genera with greatly elongated 

 rostrum) , the lachrymal bone above or in front of anterior extremity of 

 toothrow, the zygomatic plate not especially emarginate below, the 

 postorbital process indicating an evident boundary between orbit and 

 temporal fossa; no parachute membrane. 



The entire family except the members of the two following groups; 

 Oligocene to Recent; Northern Hemisphere, South America, conti- 

 nental Africa. 



Subfamily Nannosciurinae. — Like the Sciurinae but orbital region 

 abnormal, the middle of orbit behind middle of skull (rostrum short), 

 the lachrymal bone above middle of toothrow, the zygomatic plate 

 conspicuously emarginate below, the postorbital process not indicating 

 an evident boundary between large orbit and much reduced temporal 

 fossa. 



Nannosciurus of the Malay region, Myosciurus of West Africa, and 

 Sciurillus of South America (the last not seen) ; Recent. 



Subfamily Pteromyinae. — Like the Sciurinae but with a well de- 

 veloped parachute membrane present. 



The Flying-squirrels; Middle Miocene to Recent; Northern Hemis- 

 phere. 



Family Geomyidae 



Skull fossorial; zygoma robust; infraorbital foramen always at end of 

 a long canal, its orifice protected from muscle pressure by counter- 



