MICROTIN^ 389 



considerably simpler than m^ ; each has a posterior transverse 

 loop, preceded by four alternating triangles, exactly correspond- 

 ing to the hinder part of m^ ; in m^, 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 nh 

 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 



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. agresiis. 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 Cricetituz and the American 

 Wood Rats {Neotomincs), but is readily distinguished by its skull 

 and teeth. Besides the Ellobii, which are unknown in our area, 

 It includes two supergeneric groups, the Lemmi or lemmings, 

 and the Microti 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 Microtince (see Forsyth Major, Proc. Zool. 

 Soc, London, ist June 1897, 719). 



