444 MILLER AND GIDLEYI SUPERGENERIC GROUPS OF RODENTS 



enlarged distal anterior limb of the masseter lateralis superficialis, its 

 general direction parallel with zygoma. Masseter medialis arising from 

 upper margin of orbit and not passing through small infraorbital 

 foramen. 



Family Bathyergidae 



Skull and external form with conspicuous fossorial adaptations. 

 Cheekteeth extremely hypsodont, though not ever-growing; enamel 

 pattern in adult a ring with or without a reentrant fold on one or each 

 side (parallel: Octodontinae) ; number of cheekteeth ranging from | to 

 f. (In the genus, Heliophohius, with the greatest number of teeth there 

 are never more than f functional at one time; the apparent addition of 

 one tooth in the upper jaw and two in the lower jaw to the maximum 

 rodent formula is probably due to a specialized condition of the milk 

 dentition.) 



The Bathyergidae oi authors; Recent; Africa. 



Superfamily HYSTRICOIDAE 



Zygomasseteric structure as in the Bathyergoidae except: Masseter 

 medialis arising from side of rostrum and passing through large infra- 

 orbital foramen. 



LATERALIS SERIES 



Masseter lateralis the chief agent in modifying form of outer side of 

 mandible; an oblique ridge extending forward from lower border of 

 angular process usually present for attachment of this muscle. 



A. — Lachrymal hone small, forming no important part of zygomatic 

 root, its lower portion confined within orbit; lachyrmal canal closed in front 

 of orbit. 



Family Hystricidae 



Skull with no special peculiarities other than a tendency (most pro- 

 nounced in the genus Hystrix) to inflation of the rostral and frontal 

 regions; mandibular rami rather freely movable at symphysis; angular 

 process deep, neither produced backward conspicuously behind articular 

 level nor folded inward along lower margin; cheekteeth \, their enamel 

 pattern slightly removed from the simple heptamerous type, the re- 

 entrant folds narrow and not angular. 



Old World porcupines; Upper Miocene to Recent. 



Subfamily Hystricinae. — Base of upper zygomatic root over a point 

 decidedly behind the anterior extremity of toothrow; cheekteeth 

 strongly hypsodont, closed at base but without definite roots; sacral 

 vertebrae 4. 



Hystrix, Acanthion, Thecurus; Africa, southern Asia, and Malay 

 region. 



