MIMOMYS 373 



Explanation of Fig. 102. 



All the .specimens figured are from the Upper Freshwater Bed at We.st 

 RuntDii, and with the exception of the subjects of figs. 18, 21 and 22 all 

 are referred to Mirnomys intermedins Newton. 



1. Left nil, ^^ry young, with prism-fold and third outer valley intact 



{Mas. Pract. Geo!., No. 959G'). 



2. Molars of an adult right mandibular ramus (Savin Coll., No. 827.1). 



3. Right /«!, with well-developed roots (Mus. Pract. Geol., No. 959 E). 

 4, 5. Molars of two adult right mandibular rami (Savin Coll., Nos. 825.2 



and 820.1). 

 6, 6a. Right /«j, old (Mu.s. Pract. Geol., No. C.R. 959L). 

 7, 8. Molars of two adult left mandibular rami (Savin Coll., Nos. 829 and 

 828.1). 

 9. Left m.j and m.^, adult (G. While). 



10. Right ;«! and m„, adult (G. White). 



11. Left till, adult (Miis. Pract. Geol., No. C.R. 959). 



12. Right niy, adult (Savin Coll., No. 825.1). 



13. Right m,, adult; "Monkey-gravel," West Runton. 



14. Left /«!, adult (Mus. Pract. Geol, No. C.R. 959C). 



15. Right mj, old, with long roots. 



16. Right /«!, adult (Mus. Pract. Geol., No. C.R. 959Z)). 



17. Left »«,, cement spaces just closed (Miis. Pract. Geol., iVo. C.R. 



959Z»). 



18. M. majori ? Right mj, adult, with vestige of fourth outer valley 



(G. White). 



19. Right >»j, cement sjiaces just closed (Mus. Pract. Geol., No. C.R. 



9595). 



20. Left Wi, adult (G. White). 



21. M. majori? Right m^, adult (Savin Coll., No. 828.2). 



22. M. majori ? Right m-^, adult (Savin Coll., No. 645.6). 



the fourth outer valley (cf . p. 379) ; it is a feature which, although 

 frequently absent, is seen in many adult teeth of M. intermedius. 

 Another right m^ in Mr. Savin's collection (Fig. 99, 9) is imperfect 

 but important. Judging from its general appearance it probably 

 belonged to a slightly younger animal than did the tooth just 

 described; but notwithstanding this it shows a slightly more 

 advanced stage in the reduction of the primitive crown, and it 

 thus affords an illustration of that variability which is so charac- 

 teristic of vanishing structures. In it the prism-fold is large 

 but weak, and the inner part of the third outer valley is now 

 undergoing in.sulation. A left )«i (Fig. 99, 10) and a similar speci- 

 men figured by Forsyth Major (P.Z.S., 1902, 1, p. 103, text-fig. 

 13, fig. 1) carry the story a stage further, since in these the 

 prism-fold is no longer distinct, and all that remains of the third 

 outer valley is an ephemeral enamel i.slet. The open pulp cavities 

 of all the teeth described above show the pattern of adult M. inter- 

 meditis. Lastly, a specimen (Fig. 99,u,ii«) is figured for the purpose 

 of showing that as in M. savini the vanishing elements, although 

 essentially not different from those found in the older species 

 M. plioccBiiicus, have now become so ephemeral that the adult 

 or reduced pattern is brought to the grinding surface of the tooth 

 by the wear of the crown before the cement spaces become closed 



