MILLER AND GIDLEYI SUPERGENERIC GROUPS OF RODENTS 437 



Subfamily Gerhillinae. — Auditcfry bullae and entire posterior portion 

 of skull enlarged; teeth subhypsodont or hypsodont, flat-crowned in 

 adults, with opposite prisms, these tending to form transverse ridges 

 joined at median line, or, in their extreme development, to separate 

 into plates; external form saltatorial. 



The Gerhillinae of authors; Recent only', unless Trilophomy shorn, the 

 Pliocene of France is a member of the group; Asia and Africa. 



Subfamily Microtinae. — Like the more hypsodont members of the 

 subfamily Cricetinae but cheekteeth often growing from a persistent 

 pulp, the enamel pattern always consisting of (at least partially) alter- 

 nating triangles, the posterior termination of m^ and m^ never rounded; 

 squamosal with distinct postorbital ridge or process. 



The Microtinae of authors; Miocene to Recent; Northern Hemisphere. 



Subfamily Lophiomyinae. — Like the Cricetinae with tubercular, 

 slightly hypsodont teeth, but skull with temporal fossa bridged by a 

 plate formed of laminae arising from the jugal, frontal, and parietal, 

 a structure not known to occur elsewhere among rodents. 



Lophiomys; Recent; Africa. 



Family Platacanthomyidae 



Like the Cricetidae but zygomasseteric structure unusual, the infra- 

 orbital foramen of normal cricetine form, but zygomatic plate much 

 narrowed, and masseter lateralis profundus extending its line of at- 

 tachment along upper zygomatic border to side of rostrum above fora- 

 men; cheekteeth subhypsodont, the enamel pattern a modified hep- 

 tamerous with tendency to form parallel oblique cross-ridges (parallel: 

 Muscardinidae) . 



Platacanihomys and Typhlomys; Recent; Southern Asia. 



Family Rhizomyidae 



Like the Cricetidae but zygomasseteric structure unusual, the infra- 

 orbital foramen with neural portion reduced or obliterated by partial 

 or entire fusion of zygomatic plate with side of rostrum; skull and 

 external form fossorial. 



Subfamily Tachyoryctinae. — Infraorbital foramen with neural por- 

 tion reduced to an inconspicuous notch by fusion of the broad zygomatic 

 plate with side of rostrum (outline of plate below foramen usually vis- 

 ible) ; skull strongly fossorial ; cheekteeth closed at base but extremely 

 hypsodont, the enamel pattern not changing in character during adult 

 life; enamel pattern in adult consisting of 2-3 parallel curved cross- 

 ridges (the concave surface directed backward and outward in upper 

 teeth, forward and inward in lower teeth; parallel: Protechimys); 

 reduced-heptamerism evident in unworn enamel cap; external form 

 modified, though not excessively, for underground life. 



Tachyoryctes; Recent; Africa. 



