598 ON THE TEETH OF THE FIELD 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. Barrerr- 
Hamitrton, 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 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 Owniculus has seven cement-spaces. 
R L R L 
a é 
First upper molars. 
Diagram of typical teeth of Diagram of abnormal teeth of 
Microtus agrestis (for comparison). Microtus agrestis. 
This tooth is, therefore, singularly constant in its pattern, and 
the variation described in the present paper, occurring in a speci- 
men 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 
