MIMOMYS 365 



3. I Mimomys savini Hinton. 



1902. Mimonujfi intermedins Forsyth Major, P.Z.S., 1902, 1, p. 106, 



text-fij,'. 15, figs. 22, 22a., and 226 (in part). 

 1910. Miiiooniijs savini Hinton, Proc. Geol. Assoc, 21, p. 491. 



Type.—BM., No. M. 69866 (Savin Collection). A detached 

 right ?/(j ; collected by Mr. A. C. Savin ; figured by Forsyth Major 

 (P.Z.S., 1902, 1, p. 106, text-fig. 15, figs. 22, 22a, and 226). 



Type horizon and locality. — Upper Freshwater Bed (Croiuerian 

 Series), at West Runton, Norfolk. 



Range in time and space. — M. savini first appears, so far as 

 is known, in the '" .shelly sand " at East Runton, Norfolk, a 

 deposit which may be part of the Weybourne Crag. In the 

 British Museum there are from this deposit at East Runton three 

 detached examples of the m^ (all of the left side), and a fragmen- 

 tary right ramus with this tooth in place (B.M., Nos. M. 6967, 

 1, 5, and 16, and M. 6965«) ; these do not differ in any way 

 from those found in the later deposit at West Runton, where 

 the species occurs in all three divisions of the Upper Fresh- 

 water Bed ; it is abundant in the lower sandy stratum, which has 

 been so profitably worked by Mr. Savin and is common in the 

 middle peaty division ; I have a single specimen, a right m^ from 

 the overlying " Monkey Gravel." The species is not known 

 to occur in any foreign deposit. 



Characters. — Size medium ; length of lower cheek-tooth row 

 7-8 mm. In the mandible the incisor cros.ses from the 

 lingual to the labial side of the jaw between jMj and mg, and the 

 posterior root of j/ij is wholly labial to the incisor. Molars more 

 hypsodont than in the earlier species, developing roots at a 

 comparatively late period of life ; with well-differentiated enamel ; 

 the re-entrant folds partly filled with cement. ?«j with a 

 persistent prism-fold developed in connection with the third outer 

 angle; its third outer valley, intact in the youngest stages of 

 wear, but reduced in later stages, as in M. plioccBnicus, by the 

 conversion of its inner portion into an enamel islet. The reduc- 

 tion, however, takes place at a much earlier moment {i.e., at a 

 far higher level of the crown) than in M. plioccenicus, and the 

 resulting enamel islet is a much more ephemeral feature of the 

 wearing surface than in the latter species, being formed and worn 

 completely away before the molar roots have commenced to 

 develop ; an external vestige of the third outer valley usually 

 persists throughout the crown (Fig. 101, \a, 2a, and PI. XIII, figs. 

 7, 8, 10). The remaining lower cheek-teeth, m^ and m^, the 

 upper cheek-teeth and skull fragments are not distinguishable at 

 present from those of the associated species M. intermedins and 

 M. majori. 



Several very young examples of the m^ (Fig. 99, i3-io) 

 from the Upper Freshwater Bed at West Runton afiord 

 material for the investigation of the earlier stages of wear 



