MILLER AND GIDLEY: SUPERGENERIC GROUPS OF RODENTS 441 



■FOUR-CUSPED SERIES 



]\Iodifications of teeth based on an underlying quadritubercular 

 structure, the hypocone always entering into the essential mechanical 

 scheme of the crown. 



A. — Skull not specially modified; upper molars with large proioconule 

 and mefaconide, and conspicuously trenchant outer commissures, their 

 structure paralleling that of the Allomyidae in the three-cusped series. 



Family Pseudosciuridae 



Skull essentially as in the Sciuravidae but with larger infraorbital 

 foramen which may have transmitted a strand of muscle. 

 Pseudosciurus; European Oligocene. 



B. — Skull excessively fossorial; occipital region obliquely truncate, with 

 lambdoid crest moved forward nearly to level of zygomatic root; frontal with 

 short postorbital process; bony horn-cores present on rostrum in two genera, 

 absent in a third; cheekteeth highly modified from a normal heptamerous 

 structure, the grinding function of toothrow in adult almost completely 

 taken over by the greatly enlarged fourth premolar. 



Family Mylagaulidae 



General structure of skull much as in the Aplodontiidae; cheekteeth 

 f or f ; a reduced-heptamerous pattern evident in slightly worn crowns, 

 but this giving place with wear to a system of narrow longitudinal and 

 oblique lakes; molars relatively small, soon crowded out by the pre- 

 molar, an excessively hypsodont, laterally compressed tooth, closed at 

 the base, and rapidly increasing in crown length from the unworn surface 

 downward. Skeleton highly modified for underground life. 



Mylagaulus, Ceratogaulus, and Epigaulus; North American Miocene 

 and Pliocene. 



C. — Skidl without special peculiarities; infraorbital foramen moderate 

 or very large, transmitting both muscle and nerve; cheekteeth subhypsodont or 

 brachydont, their modifications based on a heptamerous structure in which 

 the ridges are narrow and the reentrant spaces wide {parallels: Funisciu- 

 rus, Erethizontidae); external form glirine or pteromyine; under side of 

 tail with scaly outgrowths near base. 



Family Anomaluridae 



Skull with moderate infraorbital foramen; lower zygomatic root at 

 levol immediately in front of anterior cheektooth; anterior point of 

 masseteric insertion on mandible beneath hinder part of nii; no dis- 

 crepancy between size of incisors and molars; cheekteeth subhypso- 

 dont, their crowns flat, longer than wide; external form pteromyine. 



Anomahirus; Recent; Africa. 



