382 MICROTIN^ 



the internal part of a highly reduced fourth outer valley, and 

 the vertical groove commencing below it is in my opinion to be 

 interpreted as the external vestige of this valley; the islet and 

 groove are fully comparable with, though much smaller than, 

 the corresponding structures that result from the reduction of 

 the third outer valley in M. flioccenicus and such later species 

 as M. savini and M. intermedius. 



Variation in the adult m-^. — The name M. majori is here used 

 for all those Cromerian forms in which the third outer valley of the 

 m-^ is not reduced by insulation of its internal portion. In the 

 type specimen (Forsyth Major, op. cit., fig. 26) this fold is deep, the 

 fourth outer prism is rather large, the fifth inner angle is obsolete, 

 and the three triangles following the posterior loop are substantially 

 closed. There is no " prism-fold " and no trace of an external 

 vestige of a fourth outer valley. The specimen has two large 

 roots, which are interesting inasmuch as they afford evidence of 

 the method by which rooted molars in Microtinee become trans- 

 formed into persistently growing or rootless ones. The cement 

 spaces on the inner side of the tooth (op. cit., fig. 13h) are not 

 completely closed below by a lip of enamel as in M. plioccenicus, 

 but are weakly continued on the bare dentine of the roots as faint 

 vertical grooves, and this downward continuation of the grooves 

 is the first step towards a downward extension of the enamel over 

 the surface of each fang. 



Fig. 105 shows the principal variations which occur in the 

 Upper Freshwater Bed at West Runton. In these teeth 

 variation is seen in the development and persistence of the " prism- 

 fold," in the size of the fourth outer and fifth inner salient angles, 

 and in the presence or absence of an external vestige of the fourth 

 outer valley. 



Two forms call for a little notice. In one, represented by the 

 second young specimen described above (Fig. 1046), by a right 

 ramus from the lower sandy division of the Upper Freshwater 

 Bed (M.P.G., No. C.R. 950c; fig. 105, 17), by a left TOj from the 

 " Monkey Gravel " (Fig. 105, 14), and by other specimens from the 

 intervening " peat " at West Runton, the m^ is characterized by 

 the rather shallow third outer valley, the absence of a " prism- 

 fold," the presence of a persistent external vestige of the fourth 

 outer valley, and by the length of the anterior loop. 



The second variety is of considerable interest, the ?% being 

 characterized by the presence of a large persistent " prism-fold " 

 of which the anterior costa is frequently strongly developed 

 (Fig. 105, I8-20). The third outer valley is quite deep and persist- 

 ent, although the great development of the " prism-fold " makes 

 it at first sight appear small. The fourth outer salient angle is 

 much reduced, and there may or may not be an external vestige 

 of the fourth outer valley. The fifth inner angle is nearly obsolete. 

 Teeth of this form have hitherto only been found in the lower 

 sandy part of the Upper Freshwater Bed at West Runton. This 



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