MIMOMYS 367 



Explanation of Fig. 101. 



All the specimens figured are from the Upper Freshwater Bed at West 

 Runton, and with the exception of the subjects of figs. 15, 16 and 17 all 

 are referred to Mimomys savini Hinton. 



1, \a. Left nil, young '*^ith enamel islet; crown and outer views {B.M., 

 No. 69C8, Savin Coll.). 

 2, 2a, 2h. Left »!j, young adult, cement spaces closing; crowrj, outer, and 

 inner views. 



3. Left »«,, young adult, cement spaces closed; outer view, PI. 



XIII, fig. 8 (Mus. Praci. Geol., No. C.R. 9597). 



4. Right TOj. 



5. Right /«!, adult; outer view PI. XIII, fig. 2 {Mus. Pract. Geol., 



No. 959i/). 



6. Right m, (0. White). 



7, 8, and 9. Molars of three mandibular rami, adult ; two right, one left 

 {Savin Coll., Nos. 821.2 (Peat), 820.1 and 821.1). 

 10, 11. Left 7«i and m^ of two adult jaws {Mit$. Pract. Geol., Nos. C.R. 

 950 and XVII 5.2.15). 



12. Left >«!, cement spaces closed, prism-fold very faint at grinding 



surface, stronger lower down (Mus. Pract. Geol., No. 959i^). 



13. Left Wi, adult; outer view, PI. XIII, fig. 3 {Savin Coll.). 



14. Right nil, adult; outer view, PI. XIII, fig. 10 {Savin Coll.). 

 15, 16. Mimomys sp. Left m^, two specimens slightly worn. 



17. Mimomys sp. Right rn-, slightly worn. 



in this species. In these young stages, the third outer angle 

 is cleft externally by a prism-fold just as in M. plioccenicus; 

 in front of the prism-fold is a deep re-entrant third outer 

 valley, partly filled with cement. On the inner side of the 

 tooth anteriorly there is a prominent fifth inner angle. So 

 far as prism-fold, third outer valley, and fifth inner angle are 

 concerned, these teeth are comparable with the second stage of 

 wear in M. jplioccBnicus (cf . Fig. 99, 2) ; but whereas in the 

 latter the cement spaces are closed below (Fig. 99, 2a), so 

 that the initial operation in the development of the roots has 

 already been performed, in these young teeth of M. savini the 

 pulps are still active and growth would have to continue for some 

 considerable time before closure of the cement spaces at the base 

 of the crown could happen (PI. XIII, figs. 7, 9). It is clearly 

 apparent from these facts that the earlier stages in the develop- 

 ment of the crown pattern have been accelerated in M. savini 

 as compared with the more ancient M. plioccenicus. 



The next stage of wear is shown by a left w^ (B.M., No. 6968, 

 Savin Coll., West Runton; Fig. 101, i, i«) in which there is no 

 sign of rooting, the pulp cavities and cement spaces being still 

 widely open below. In this tooth, which has the persistent prism- 

 fold characteristic of the species, the third outer valley is already 

 reduced, its inner part being represented by an enamel islet and 

 its outer portion by a groove which, in this specimen, extends 

 from the present .summit half-way to the base of the tooth. In 

 pattern therefore the tooth is similar to a middle-aged »ij of 

 M. plioccenicus (cf. Fig. 99, 4). In front of the external vestige 



