IJUPLICIDENTATA 483 



ARTIFICIAL KEY TO EUROPEAN MEMBERS OF 

 RODENT FAMILIES. 



(Based exclusively on the teeth.) 



Incisors ~^^; cheek-teeth ''[^ Lrporichi-, p. AS-i. 



Incisors \'_\ ', cheek-teeth not more than '^'• 

 Cheek-teeth 'j^^. 



Crowns of mandibular cheek-teeth with con- 

 spicuous median concavity Sciurid;>\ p. 897. 



Crowns of mandibular cheek-teeth without 



conspicuous median concavity Petauristid.T, p. 940. 



Cheek-teeth less than ^"j. 

 Cheek-teeth j^. 



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



Cheek-teeth hypsodont, rootless. 



Enamel foldings transverse only Casfor'uh-, p. 947. 



Enamel foldings transverse, oblique and 



longitudinal Hystricid.-r, p. 542. 



Cheek-teeth less than ^. 



Cheek-teeth .p, Zapod'uhv,-^. 5.35. 



Cheek-teeth Z~^^. 



Enamel foldings sigmoid Spalacidx, p. 887. 



Enamel foldings not sigmoid Mtiridx, p. 591. 



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



Cheek-teeth brachyodont, tubercular. 

 Tubercles of upper molars arranged 



in two main rows Crlcetinx, p. 592. 



Tubercles of upper molars arranged 



in three main rows Marinve, p. 701. 



Sub-Ordkr duplicidentata. 



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



Geographical distribution. — Essentially that of the oi'der 

 Modentia except that it does not include Madagascar or Australia. 

 One genus now almost universally distributed in the warmei" 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 tlie same time, the 

 motion of the jaws in mastication consequently lateral ; premolars 

 ;; ■; ; 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. 



ItemarJcs. — The well defined sub-order* Dtiplicidentata contains 



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

 reason to l)elievo that the resemblances between the two groups are the 

 result of convergence rather than of relationship. 



2 I 2 



