370 MICROTIN^ 



as follows : m^ with two distinct roots, viz., a large anterior and 

 a smaller posterior fang. An examination of about 100 specimens, 

 all from West Runton and all with more or less well-grown roots, 

 showed clearly that the anterior fang in this species is a com- 

 pound structure consisting of the anterior fang proper and a 

 coalesced, though still clearly recognizable, representative of the 

 intermediate root which supports the second inner prism in this 

 species as in M. plioccenicus ; in the latter species, as described 

 above, this intermediate fang is distinct from, and not fused with, 

 the front root. Both irfi and m^ have normally two roots each in 

 M. intermedius ; among a very large number of examples of the m^ 

 from West Runton, old enough to show more or less well-devel- 

 oped roots, I found that the anterior root varied, being sometimes 

 considerably and sometimes only a little larger than the posterior 

 fang. Two specimens (a left m^ from the lower sandy division of 

 the Upper Freshwater Bed, and a left m- from the overlying peaty 

 stratum at West Runton, are remarkable for showing a third 

 quite distinct but very small root, which supports the second 

 inner prism; this exceptional third root is not the homologue 

 of that normally occurring in the m^ of M. plioccenicus, for in 

 that species the third root supports the inner part of the 

 anterior loop (Fig. 100, isa). In the lower jaw each tooth develops 

 two roots, an anterior and a posterior. 



In enamel pattern the adult cheek-teeth of M. intermedius are 

 almost exactly like those of most species of Arvicola. In the upper 

 jaw m^ and m^ agree with those of Arvicola and other normal 

 voles, although when little worn or unworn (Fig. 101, 15-17) they 

 show ephemeral complications which are of great interest 

 because of their bearing upon some of the problems connected 

 with the evolution of the Microtine dentition, a matter discussed 

 above (p. 102). Although subject to a good deal of variation 

 in small points, all the very numerous examples of the ?n^ 

 from West Runton examined by me agree in one important 

 respect ; in this tooth the second inner fold persists as a normal 

 fold throughout the crown instead of being subject to reduction by 

 insulation as in M. plioccBuicus (Fig. 99, 24-26). This character 

 alone suffices to prove that M. intermedius, and its allies in the 

 Upper Freshwater Bed, are not the descendants of M. plioccenicus, 

 but must have arisen from some unknown species no doubt 

 related to it but differing in the less reduced form of the m^. 



It must be understood that the foregoing description of 

 the skull and the maxillary dentition is based upon the remains 

 of all three species {M. intermedius, M. savini, and M. majori) 

 found associated in the Upper Freshwater Bed at West Runton. 

 With the fragmentary though abundant material at present 

 available no means of distinguishing between these species other 

 than by the characters afforded by the m^ have as yet been 

 discovered. 



In adult stages of wear the lower cheek-teeth agree closely 



