Dental Formula of the Muridae. 1C7 



jaw in the most primitive members of the family; and it is a 

 complexity of wliieh traces are retained to tlii.s day by many 

 genera distributed among several of the most widely separated 

 living snbfamilies. 



Our knowledge of Tertiary Rodentia is still very imperfect, 

 for few have been discovered beyond the limits of Europe 

 and America. On the other hand, those of Europe and 

 Nortli America are fairly well known. In the Tertiarj'- 

 strata of these two continents the Muridie are comparatively 

 poorly represented, and the known forms include none that 

 can be regarded as even approximately representing the 

 atavus of this great family. Upon reviewing all the molar 

 types so far discovered among the earlier and middle Tertiary 

 rodents, we fail to find one from which it would be possible 

 to derive cheek-teetb possessing the complexity so charac- 

 teristic of " nii'^ and " m^ " in so many Muridae, living and 

 extinct, dating from the Pliocene period onwards — unless, 

 indeed, we make the at present quite inadmissible sup- 

 position that new parts have been added to the teeth since 

 the Miocene period. Such a sup])osition is inadmissible, 

 firstly, because it ^ould contradict all that we have learnt 

 about the direction of dental specialization within the 

 Muridae from Pliocene times onward ; and, secondly, because 

 it would traverse the evidence of the older Tertiary rodents 

 themselves. For even in the Eocene and Miocene periods 

 rodent molars have generally undergone a process of progres- 

 sive simplification {e.g., Lagomorpha *) ; and to no order 

 does Kowalevsky's famous comment ("Das kann schon als 

 ein Wink dienen, wie weit wir noch von der primitiven Form 

 des Zahnes sind, denn je tiefer wir in den Schichten dringen, 

 je altere Formen wir finden, desto complicirtere Gestalten 

 tauchen immer auf ''"'t) — uttered in another connection, it is 

 true — apply with greater force. 



The seeming impossibility of deriving these complex teeth 

 from any known rodent permanent molar or premolar led, 

 naturally enough, to the examination of the milk-dentition 

 as the possible source of these organs. In most mammals 

 the posterior milk-molars (and particularly mp^) are com- 

 monly more complex than are either their vertical successors 



{p. J or the true molars [m.^^) behind them. And among 



Tertiary rodents this is also sometimes the case, as, for 



* Cf. Forsvth Major, '' On Fossil and Recent Lagomorpha,''Tr. Linn. 

 See. Loudon, "(2) vii. p. 433 (1899). 



t Kowalevsky, " Anthracotherium," PaLneontograpliica, xxii. p. 230, 

 footnote (1874)." 



