118 MICROTIN^ 



the lower molars are the homologues of the anterior loops of ni^ 

 and w^. In m^ the anterior part of the tooth is more complex ; 

 cusp x^, owing partly to the terminal position of the tooth in 

 the tooth-row, partly to the fact that the cusp is the opponent of 

 the complex anterior loop of j>?i, is large and persists as an essential 

 element of the crown. In the other teeth this cusp has been 

 suppressed ; but small complications of the anterior loop in m^ 

 and m^ and of the posterior loops of the lower molars may be seen, 

 at all events occasionally, in all genera. These features when 

 present always occur, as will be seen from the figures, in definite 

 situations, and on the view of the homologies here adopted they 

 are easily interpreted as being vestiges of the cusps x^ and x ; but 

 if any other view of the homologies be taken they are meaningless. 

 In the unworn m^ of Dicrostonyx cusp x^ is separated from cusp 1 

 by a little V-shaped median and longitudinal valley which furrows 

 the anterior surface of the tooth (Fig. 57). More or less clear 

 traces of the separation of those two cusps may be seen in many 

 genera from time to time. Of the median tubercles persistent 

 traces of y and z, representing respectively the " protoconule " 

 and " metaconule " of Trituberculy, frequently occur in the upper 

 molars of Microtinse ; such a vestige of y is one of the normal 

 features in the upper molars of the genus Lagurus (Fig. 43). 



In ?rt^ we find a striking parallel to m^. The posterior part of 

 the tooth, corresponding with the anterior part of wij (and no 

 doubt owing to the fact that it, like the latter, develops freely, 

 unhampered by the presence of any neighbour), shows a great 

 range in complexity. In many genera as in Anteliomys (Fig. 88) 

 some species of Evotoinys (Fig. 78) and AUicola (Fig. 91, la), it is 

 very complex ; but from that most complicated condition a 

 continuous gradation of forms leading down to the most reduced 

 and simplified condition, found in Prometheomys and Elldbius, may 

 be traced (Figs. 55, 56, 91, i-i6, 95). In some genera as in Arvicola, 

 Evotomys and some species of Microtus (e.g., 31. nivalis) where the 

 adult 7>i^ is more or less extensively simplified longitudinally, 

 young or unworn specimens of this tooth show more or less 

 ephemeral additional salient angles on one or both sides behind. 



In species in which the cheek-teeth develop roots in old age 

 the changes seen in the enamel pattern as the teeth are worn down 

 are most instructive. In Evotomys riifocanus (Figs. 10, i, 2, ii ; 81, 

 82) «ii and m^ are characterized by their greater complexity in 

 youthful stages of wear ; by middle life the patterns of these teeth 

 are much simplified and the dentinal spaces in all the molars are 

 tightly closed, the teeth having a form recalling Microtus rather 

 than Evotomys; but after the roots are developed, in extreme 

 old age, the re-entrant folds become shallow, the salient angles 

 rounded, and the teeth acquire a form which resembles that seen 

 in any ordinary and more primitive species of Evotomys. Here 

 we have an illustration of the truth contained in that second 

 paragraph quoted from Ameghino on p. 104. 



