MICROTIN “& 389 
considerably simpler than 7, ; each has a posterior transverse 
loop, preceded by four alternating triangles, exactly correspond- 
ing to the hinder part of wz,; in mz, in front of the fourth 
triangle, and but partially shut off from it, is a vestigial pair of 
prisms of which the outer member is the more reduced; in 772g 
the reduction of this vestigial pair is carried still further, and 
the fourth triangle itself is quite small. Each tooth has thus 
five dentinal spaces, three salient angles and two infolds on 
each side, and in addition is complicated by the presence of 
the vestigial structures described. 
Dental formula:—Forsyth Major long ago argued that 
the dental formula of the cheek-teeth should be written as 

I ine . . 
dm—, m ——, the anterior cheek-teeth being regarded as per- 
sistent milk molars. This view has recently been revived by 
Hinton; it is based upon the extraordinary complexity of the 
anterior cheek-teeth and upon various theoretical considerations, 
and support is lent to it by an instance recorded by Winge of 
the occurrence of a small fourth posterior cheek-tooth in 
M. agrestis. In describing the teeth here the older notation 
is retained as being more convenient. 
The sub-family is circumpolar, Reaching its main develop- 
ment in temperate climates, it ranges south to the northern 
coasts of the Mediterranean, northern India and Mexico, and 
north to the limits of mammalian life. It first appears in 
the Pliocene of Europe, but in America not before the 
Pleistocene, so it may have originated in the Old World. 
It is closely related to the Crrcetine and the American 
Wood Rats (Veotomine), but is readily distinguished by its skull 
and teeth. Besides the “//oézz, which are unknown in our area, 
it includes two supergeneric groups, the Lemmz or lemmings, 
and the MWZicrotz or voles, the former extinct in Britain. It 
possesses great interest from the primitive characters by which 
it seems to be connected with the Malagasy Brachytarsomys, a 
genus which, so far as structure goes, might itself have been 
a forerunner of the MZzcrotzne (see Forsyth Major, Proc. Zool. 
Soc., London, 1st June 1897, 719). 
