INTERRELATIONSHIPS OF GENERA 47 



adult stages of wear. But as we trace the genus onwards from 

 earlier to later horizons we observe that the species become more 

 and more hypsodont, the growth of the molars ceasing and roots 

 being developed at later moments in the life of the individual 

 in the more modern species than in the earlier members of the 

 genus. Simultaneously the teeth undergo a process of simplifi- 

 cation, visible not only as we trace the genus forward from one 

 geological horizon to another, but as we follow the development 

 or wear of the individual tooth from infancy to age. The genus 

 thus presents us with some beautiful examples of recapitulation 

 which are described in detail below (p. 111). The most interest- 

 ing tooth, as always in voles, is wij. In the most primitive forms 

 it no doubt resembled the corresponding tooth of Dolomys in 

 possessing not fewer than five closed triangles between the 

 posterior and anterior loops ; in addition the third outer 

 salient angle was complicated by a large vestige of one of the 

 tubercles of the primitive median row giving rise to a peculiar 

 feature which I have called the " prism-fold." In early species, 

 like M. plioccenicus, the third outer re-entrant fold becomes 

 reduced in early middle age by the conversion of its internal 

 portion into an enamel islet, and this islet and the " prism-fold " 

 persist until a very advanced stage of wear has been reached. 

 When very young the m^ of a late species, e.g., M. intermedins, 

 resembles a young m-^ of M. plioccenicics in all essential respects. 

 In exactly the same way it is reduced by the conversion of the 

 third outer fold into an enamel islet, and as wear proceeds the 

 islet disappears and a simplified pattern showing only three 

 triangles between the posterior and anterior loops is brought 

 to light on the surface of the crown. This simplified pattern is 

 the characteristic adult pattern of the tooth in the later species, 

 and it is developed and perfected long before the tooth ceases 

 to grow. The archaic elements in the later species are confined 

 to the apical portions of the crown, and the process of reduction, 

 though in every detail a faithful repetition of that seen in M. 

 flioccenicus, is performed within the first few weeks or days of 

 the individual's existence instead of occupying most of its life 

 as in the older form. 



In adult stages of wear the cheek-teeth of the later species of 

 Mimomys are not to be distinguished from those of Arvicola by 

 pattern alone, and as the general progress is towards hypsodonty 

 and persistent growth there comes a moment in the geological 

 scale when, with existing materials, it is impossible to say 

 definitely whether we are dealing with Mimomys or with Arvicola. 

 If the teeth could be shown to develop roots in old age they would 

 be referred to the former genus; but as no sign of roots, not 

 even an incipient closure of the cement spaces or pulp cavities, 

 can be detected, the remains in question, from the later Cromerian 

 beds of Bacton and the early Middle Terrace deposits of the 

 Thames, have to be referred to Arvicola. 



