DUPLICIDENTATA 483 



ARTIFICIAL KEY TO EUROPEAN MEMBERS OP 

 RODENT FAMILIES. 



(Based exclusively on the teeth.) 



Incisors j|; cheek-teeth jg Leporidie, p. 484. 



Incisors j~j ; cheek-teeth not more than jj|- 

 Cheek-teeth ~. 

 Crowns of mandibular cheek-teeth with con- 

 spicuous median concavity Sciuridie, p. 897. 



Crowns of mandibular cheek-teeth without 



conspicuous median concavity Petauristidir , p. 940. 



Cheek-teeth less than ^. 

 Cheek-teeth ^- 



Cheek-teeth brachyodont, rooted Muscardinidie, p. 549. 



Cheek-teeth hypsodont, rootless. 



Enamel foldings transverse only Castoridm, p. 947. 



Enamel foldings transverse, oblique and 



longitudinal Hystricidx, p. 542. 



Cheek-teeth less than £j. 



Cheek-teeth g Zapodidm, p. 535. 



Cheek-teeth £f. 



Enamel foldings sigmoid Spalacidx, p. 887. 



Enamel foldings not sigmoid Muridee, p. 591. 



Cheek-teeth hypsodont, prismatic Microtinx, p. 610. 



Cheek-teeth brachyodont, tubercular. 

 Tubercles of upper molars arranged 



in two main rows Cricetinx, p. 592. 



Tubercles of upper molars arranged 



in three main rows Murinne, p. 791. 



Sub-Oedeb DUPLICIDENTATA. 



1811. Duplicidentata Illiger, Prodr. Syst. Mamm. et Avium, p. 91 



Geographical distribution. — Essentially that of the order 

 Bodentia except that it does not include Madagascar or Australia. 

 One genus now almost universally distributed in the warmer por- 

 tions of both hemispheres as the result of artificial introduction. 



Characters. — -Upper incisors 2-2, their enamel covering 

 extending to posterior surface, the second tooth minute, sub- 

 terete, without cutting edge ; distance between mandibular 

 tooth-rows much less than that between maxillary tooth-rows, 

 only one pair of rows capable of opposition at the same time, the 

 motion of the jaws in mastication consequently lateral ; premolars 

 H ; incisive foramina very large, confluent posteriorly ; bony 

 palate reduced to a narrow bridge lying mostly between the 

 premolars ; facial portion of maxillary cribriform or incomplete ; 

 fibula articulating with the calcaneum. 



Bemarks. — The well defined sub-order* Duplicidentata contains 



* Not improbably an order distinct from the Rodentia, as there is 

 reason to believe that the resemblances between the two groups are the 

 result of convergence rather than of relationship. 



2 I 2 



