EVOLUTION OF MOLARS 111 



Arvicola, we do glean some additional information from the 

 less-worn teeth that are available, for certain of the highly- 

 ephemeral complications seen in the young Arvicola are more 

 strongly developed and persist to a far later stage of wear in some 

 of the species (and particularly the geologically older species) of 

 Mitnomys. 



The oldest known form is Mimomys flioccBnicus from the 

 Norwich Crag, and in Fig. %Oa-e, the m^ of this species 

 is shown in different stages of wear. In this genus the molars 

 develop roots in adult life and by comparing the degrees to which 

 the roots are developed we are able to form an idea of the relative 

 ages of the teeth before us. In Fig. 60, a represents a right m^ in 

 the youngest stage of wear yet seen ; but the lateral view, a , 

 shows that the tooth belonged to a subadult individual, since the 

 cement spaces are already closing below, the first step towards the 



7 6 7 Q \C/^-\2/ X' 



Fig. 59. — Right upper molars of Arvicola amphibius Linnseus, lettered 

 to show the homologies of the cusps. 



In this specimen w^ shows a well-marked vestige of cusp z (or " meta- 

 conule ") between 4 and 7; a weaker trace of the same structure and 

 traces of other median tubercles are seen in j/i^. 



development of roots; b and b' represent a slightly older 

 tooth in which the cement spaces are completely closed below ; 

 c and d represent two teeth which belonged to individuals in the 

 prime of life ; lastly, e represents a very old specimen provided 

 with very long fangs, its crown being almost worn away. 



If we compare the youngest though considerably worn tooth 

 of M. plioccBnicus (Fig. 60o) with the unworn Wj of Arvicola (Fig. 

 58c) we see, on making due allowance for the difference in in- 

 dividual age, a very striking resemblance. In each the posterior 

 loop is followed by five substantially closed triangles and a small 

 anterior loop ; in each the third outer fold is deep and is directed 

 backwards instead of forwards. The third outer prism in each is 

 cleft by a vertical " prism-fold," very shallow and superficial, it 

 is true, in the tooth of modern Arvicola (in which it rarely occurs), 

 deep and persistent throughout life in M. plioccenicus ; this 

 " prism-fold " incompletely separates the outer tubercle, which 

 forms the tip of the salient angle, from the median tubercle (very 

 conspicuous in Mimomys, less so in Arvicola) which lies in front 



