EVOLUTION OF MOLARS 115 



of Jrtj is reduced by insulation ; but although some early Pleistocene 

 species of Arvicola appear to agree with these species of Mimomys 

 in this respect, others, including all the known later Pleistocene 

 and recent species, do not, for in these later forms, as described 

 above, the fold in question is reduced merely by a gradual weaken- 

 ing or transverse shallowing. Side by side, however, there occur 

 in the same deposits with those of M. plioccpnicus, M. interniedius 

 and M. savini the remains of other species of Mimomys which seem 

 to stand nearer to the ancestor of the existing species of Arvicola. 

 In these {M. newtoni and M. majori) the third outer fold of m^, 

 although often reduced in depth as in modern Arvicola, never 

 suffers reduction by insulation. The material representing the 

 older species, M. newtoni, is still too scanty for satisfactory dis- 

 cussion, but remains of the later form, M. majori, are more 

 abundant in collections. The youngest specimen I have seen is 

 represented in Fig. 1046, and in PI. XV, fig. 1, and it is a 

 tooth of considerable interest. It shows that confluency of the 

 dentinal spaces which commonly appears in the teeth of young 

 voles shortly after the removal of the tubercular cap by wear. 

 There is no trace of the " prism-fold " ; but the third outer 

 valley, already containing a little cement, is persistent. In front 

 of this fold and obliquely placed upon the outer margin of the 

 anterior loop is a small enamel islet which represents the inner 

 part of a highly reduced fourth outer fold. Just below the swollen 

 outer lip of the islet there commences a shallow furrow which 

 passes vertically down the outer side of the crown. If the islet 

 had not been present it would have been difficult to interpret the 

 meaning of the furrow, and before the discovery of this young 

 tooth the furrow, observed in many adult examples of the ??ii of 

 M. majori, puzzled me considerably. But the relation of the 

 furrow to the islet shows quite clearly that the former must be 

 regarded as the external vestige of the reduced fourth outer valley. 

 This vestige is often present and is long persistent in M. majori 

 as shown by the figures in PI. XV ; on the grinding surface, the 

 only feature that it makes is a minute curl in the enamel of the 

 outer border of the anterior loop (cf. Fig. 105, ii). 



Attention may be called also to the young tooth shown in 

 Fig. lOda and a ; this specimen, referred to typical M. majori, 

 closely resembles the young m^ of M. intermedins (Fig. 61a) in 

 general form ; it shows not only a persistent third outer fold 

 and a persistent external vestige of the fourth outer fold, but 

 very clear traces of the median tubercles y and z. 



A very young m^ of M. intermedius (Fig. 101, lo) shows 

 that transverse passes across the summit of the crown, described 

 above as occurring in unworn teeth of Arvicola, occur also in 

 Mimomys; the last trace of one is seen crossing the isthmus 

 between the two posterior triangles. 



The unworn )«j of 0)idatra (Fig. 58«) shows clearly that in 

 this genus the enamel does not extend across the summit of the 



