MILLER AND GIDLEYI SUPERGENERIC GROUPS OF RODENTS 447 



hypsodont, but enamel structure not completely multilaminar; exter- 

 nal form robust, terrestrial. 



Cuniculus { = "Coelogenys"); Tropical America; Pleistocene and 

 Recent. 



Family Heptaxodontidae 



First tooth of maxillary series mechanically dominant, cheekteeth 

 apparently reduced to -|, conditions not known elsewhere in the Hijstri- 

 coidae, and indicating zygomasseteric development along a line different 

 from that followed elsewhere in the group; enamel structure multi- 

 laminar with reduplication in the anterior tooth; diagnostic cranial 

 characters unknown. 



Heptaxodon; Porto Rico; Pleistocene? The genus Morenia from the 

 South American Miocene may be a second member of the family; it is 

 at present known from isolated teeth only. 



'B.— Lachrymal hone large, usually forming an important part of zygo- 

 matic root, its lower portion extending forward out of orbit to a level in front 

 of anterior margin of infraorbital foramen; some part of lachrymal canal 

 open on side of rostrum in front of orbit. 



Famil}' Dasyproctidae 



Skull generalized in structure, closely resembling that of the less 

 specialized Hystricidae; cheekteeth hypsodont but with a nearly unmod- 

 ified heptamerous structure, paralleling that in the Hystricidae; ex- 

 ternal form cursorial, the legs lengthened, the digits 5-3.^'' 



The Dasyproctidae of authors with Cuniculus removed and Neo- 

 reomys added; South and Middle America; Miocene to Recent. 



Family Chinchillidae 



Cheekteeth with heptamerous structure excessively modified, the 

 enamel pattern consisting of parallel transverse laminae (parallel: 

 Dinomyidae) ; mandible with no sharply defined ridge for attachment of 

 masseter lateralis; external form saltatorial. 



South America; IMiocene to Recent. Living genera: Chinchilla, La- 

 gostomus, Viscaccia. Extinct genera: Euphilus, Perimys, Pliolagosto- 

 mus, Prolagostomus, Scotaeumys, Sphaeromys. 



Family Abrocomidae 



Like the Chinchillidae but cheekteeth with deep reentrant angles on 

 both sides, and mandible with sharply defined ridge for attachment of 

 masseter lateralis; external form not saltatorial. 



Abrocoma; South America; Pliocene to Recent. 



"> The feet of Neoreomys are imperfectly known, but there appears to be noth- 

 ing in the structure of the parts which have been described that indicates the 

 presence of more than three digits in the hind foot. 



