434 MILLER AND gidley: supergeneric groups of rodents 



sinking in an oblique sulcus; frontal without postorbital process; cheek- 

 teeth evenly hypsodont or in their extreme development ever-growing, 

 the fundamental tritubercular plan lost in functional adult teeth, the 

 first and second molars of adult consisting of either one or two simple 

 loops. External form in living members of the group highly modified 

 for underground life. 



Subfamily Entoptychinae. — Angular portion of mandible mostly 

 below alveolar level; cheekteeth rooted, the enamel pattern of first and 

 second molars consisting of two simple loops joined at protomere.^ 



Entoptychus; North American Oligocene. 



Subfamily Geomijinae. — Angular portion of mandible mostly above 

 alveolar level; cheekteeth ever-growing, the first and second adult 

 molar consisting each of a simple prism, with an enamel plate always 

 present on anterior surface in upper teeth and on posterior surface of 

 lower teeth. 



North American pocket gophers; Miocene to Recent. 



Family Heteromyidae 



Essential characters as in the Geomyidae but skull not fossorial; 

 zygoma slender; orifice of infraorbital canal protected from muscle 

 pressure by countersinking in a vacuity which extends transversely 

 through rostrum; external form murine or saltatorial. 



North American pocket-mice and kangaroo-rats; Middle Ohgocene 

 (Heliscomys) to Recent. 



FOUR-CUSPED SERIES 



Teeth becoming hypsodont on the basis of a quadritubsrcular 

 structure. 



Family Adjidaumidae 



Zygomasseteric structure^ and infraorbital canal as in the Sciur- 

 idae; cheekteeth |-, slightly hypsodont, the enamel pattern unmodified 

 heptamerous.* 



Adjidaumo; North American Middle Oligocene. 



2 Protomere = inner side of maxillary cheekteeth and outer side of mandibular 

 cheekteeth. 



Paramere = outer side of maxillary cheekteeth and inner side of mandibular 

 cheekteeth. 



' Zygomasseteric structure = the combined and correlated structures of the 

 masseter muscle and of the skull in the region at which the muscle takes its origin. 



* Heptamerous pattern = the enamel pattern of a flat-crowned cheektooth in 

 which each of seven original tubercles is represented by a loop (two on the proto- 

 mere, five on the paramere). 



