368 MICROTIN^ 



of the third outer valley there is another narrower, fainter, but 

 long persistent groove, which I interpret as the last feeble vestige 

 of a much more reduced fourth outer valley (see p. 379). 



In slightly older specimens than the oue just described we 

 meet with the typical adult pattern of M. savini. Such examples 

 (Fig. 101, 2) show that the enamel islet is worn out and disappears 

 before the closure of the cement spaces is effected below ; such a 

 pattern is developed in M. plioccenicus only in the latest stages of 

 wear when the individual is on the threshold of senility (cf. 

 Fig. 100, 2). Still older examples of the m^ of M. savini (PI. XIII, 

 figs. 8, 10), with closed cement spaces, prove that the prism- 

 fold is always persistent, and that the external vestige of the 

 third outer valley is usually persistent throughout the crown 

 in this species. The only change of pattern which takes place 

 rather regularly in the closing stages of wear is that the fifth 

 inner angle tends to become obsolete in old age (Fig. 101, 9). 



Typically in M. savini the prism-fold is narrow and the 

 external vestige of the third outer valley wide ; in the crown 

 view, the emargination formed by the latter structure usually 

 makes a nearer approach to the middle line of the tooth 

 than does that formed by the prism-fold. As in M. flioccBnicus 

 the relative antero-posterior widths of the two structures in ques- 

 tion may vary considerably; in some specimens (Fig. 101, 13) the 

 prism-fold becomes very wide at the expense of the valley vestige, 

 and I have seen a few teeth in which the latter disappears before 

 reaching the general base of the crown (Fig. 101, \a). Occasionally 

 the anterior costa of the prism-fold is but feebly developed, and 

 in such cases it is sometimes difficult to distinguish jaws of this 

 species from those of M. intermedius (Figs. 101, 12). 



4. t Mimomys intermedius Newton. 

 1881. Arvicola (Evotomys) intermedius Newton, Geol. Mag., [2], 8, 



p. 258; Vert. Forest Bed, 1882, p. 83, pi. xiii, figs. 3, 3a, and 36 



(in part). 

 1890. Microtus intermedius Woodward and Sherborn, Catal. Brit. Foss. 



Vert., p. 366 (in part). 

 1902. Mimomys intermedius Forsyth Major, P.Z.S., 1902, 1, pp. 103- 



105 (in part). 

 1910. Mimomys intermedius Hinton, Proc. Geol. Assoc, 21, p. 491. 

 1914. Microtomys intermedius Mehely, Ann. Mus. Nat. Hungar., 12, 



p. 211. 



Type. — Museum Practical Geology; an adult right mandi- 

 bular ramus with all the teeth in place, collected by Clement Reid, 

 F.R.S. Figured by Newton, Vert. Forest. Bed, 1882, PI. xiii, 

 figs. 3, 3a and 3fc, and in the present work (Fig. 99, 12). 



Type Jwrizon and locality. — Upper Freshwater Bed, Cromerian, 

 Late Pliocene ; West Runton, Norfolk. 



Range in time and space. — In Britain remains of M. intermedius 



