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Show 598 ON THE TEETH OP THE EIELD VOLE. [May 19, The following papers were read:- 1. On a Variation in the Pattern of the Teeth of a specimen of the Common Field Vole. By G. E. H. BARRETT-HAMILTON, F.Z.S. [Eeceived 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 first 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 Euotomys, Synaptomys, Myodes, Fiber, Neofiber, and Ellobius. In Siphneus the pattern is indistinct, and Cuniculus has seven cement-spaces. R L R L First upper molars. Diagram of typical teeth of Diagram of abnormal teeth of Microtus agrestis (tor comparison). Microtus agrestis. This tooth is, therefore, singularly constant in its pattern, and the variation described in the present paper, occurring in a specimen of Microtus agrestis, 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 (6) 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 |