598 ON THE TEETH Oh' THE FIELD A'OLB. > [Mliy 19, 



The following papers were rend : — 



1. On a Variation in the Pattern of the Teeth of a specimen 

 of the Common Field Vole. By G. E. H. Barkett- 

 Hamilton, F.Z.S. 



[Received April 27, 1896.] 



The pattern of the molar teeth of the Voles has always been 

 regarded as an important feature in the classification of these 

 animals. Of these teeth the iirst lower and third (last) upper 

 show the most important specific and subgeneric characters. The 

 remainder vary much less among the various species and subgenera 

 than do the above, and of these the first upper is undoubtedly the 

 most constant. As will be seen from the figure (a), the first upper 

 molar has five cement-spaces with three external and three internal 

 angles. The first cement-space is placed anteriorly, the second and 

 fourth on the inside, and the third and fifth on the outside of the 

 tooth. This is the form of the tooth throughout the genus Microtus, 

 and the same pattern occurs also in the allied genera Euntomys, 

 Synaptomys, Myodes, Fiber, Ncojiher, and Ellohius. lu Sii'Iineics the 

 pattern is indistinct, and Cuniculus has seven cement-spaces. 



R 



a b 



First upper molars. 



Diagram of typical teeth of Diagram of abnormal teeth of 



Microlim ayrentis (for comparison). Microtus agrcdis. 



This tooth is, therefore, singularly constant in its jiattern, and 

 the variation described in tiie present pa])er, occurring in a speci- 

 men of Microtus ayresHs, is on that account of some interest, as 

 there is little doubt that had the specimen been received from 

 some unknown or distant region, it would probably have formed 

 the basis of a new species or perhaps even of a new subgenus. 

 The variation, which occurs in the first upper molar on each side, 

 is well shown in the figure (b) and requires only a few words of 

 description. It consists of an extra small, but distinct internal 

 cement-space, formed by an additional folding inwards of the 

 enamel. The first molars in this specimen have therefore six 



