MILLER AND GIDLEY: SUPERGENERIC GROUPS OF RODENTS 435 



Family Eutypomyidae 



Like the Adjidaumidae but with cheekteeth somewhat more hypso- 

 dont and the heptamerous enamel pattern complicated by the devel- 

 opment of a considerable number of secondary closed loops which ap- 

 pear in partially worn teeth as an aggregation of minute enamel lakes 

 covering nearly entire surface of crown. 



Eutypomys; North American Middle Oligocene. 



Family Chalicomyidae 



Like the Adjidaumidae but cheekteeth strongly hypsodont and 

 enamel pattern reduced-heptamerous (sometimes paralleling that of 

 the Hystricidae) becoming rapidly simplified as the crowns wear away; 

 skull occasionally fossorial; no postorbital process on frontal; no pit- 

 like depression in basioccipital region. 



ChaUcomys (= Steneofiber) and related genera, European Miocene 

 and Pliocene; Trogontherium, European Pliocene and Pleistocene; 

 Palaeocastor, Eucastor and related genera, North American Upper 

 Oligocene and Lower Pliocene. 



Family Castoridae 



Skull with rostrum broadened and deepened and braincase narrowed; 

 basioccipital region with conspicuous pit-like depression ; cheekteeth not 

 ever-growing but so excessively hypsodont that the slightly reduced- 

 heptamerous pattern (parallel : Myocastor) changes little with age and 

 rarely if ever wears out; external form highy modified for aquatic life; 

 caudal vertebrae flattened. 



Castor; Lower Pliocene to Recent; Northern Hemisphere. 



Family Castoroididae 



Zygomasseteric structure modified by the passage of the shaft of 

 the incisor below the infraorbital foramen instead of above it, the ridge 

 formed by the tooth dividing the area of masseteric origin on side of 

 rostrum into two planes; posterior nares divided horizontally by the 

 median fusing of palatine bones over roots of cheekteeth; teeth ever- 

 growing, the enamel pattern a series of 5-7 parallel transverse ridges 

 (parallel: Dinomyidae). 



Castoroides; North American Pleistocene. 



Superfamily MUROIDAE 



Zygomasseteric structure as in the Sciuroidae except: Lifraorbital 

 foramen superior in whole or in part, entered or traversed by muscle 

 as \vell as nerve; masseter lateralis seldom reaching superior border of 

 rostrum, and never doing this to exclusion of masseter medialis. 



THREE-CUSPED SERIES 



Modifications of teeth based on an underlying tritubercular structure. 



