EVOTOMYS 403 



The posterior loop may be simple or complicated by the 



presence of extra infolds — generally a shallow antero- 



external and a deep internal^ — the 



latter extending across the tooth 



nearly to the enamel of the outer *«fl^ — mi — 



side ; there are many intermediate 



conditions, but in the simplest form i^gj^ — mi — 



there are on each side three salient 



angles and two infolds ; in the most -^^ _ ^, 



complicated form there are four or 



five salient angles and four infolds on 



each side. 



, . Fig. 62.— Right Cheek-Teeth 



In m-^ the posterior transverse loop of Evotomys gianoius (a, upper ; 

 is preceded by two outer and three ^' '°"'" : "°™" ^'^w ; 7 ''"«= 



life size")* 



inner more or less closed triangles ; 



the tooth has thus four outer and four or five inner salient 

 angles ; the third inner triangle opens more or less broadly 

 into the short anterior loop, which presents externally a well- 

 marked salient angle, while its inner border may form another 

 salient angle or may be rounded and only slightly salient. 



m^ and m^ are composed each of three more or less trans- 

 verse dentine-loops, giving rise to six salient angles and four 

 infolds ; those of the outer side are poorly developed, espe- 

 cially in the m^, in which when well-worn the anterior angle 

 and infold may be absent. The first and second loops of m^, 

 and the second loop of m^, especially the former, may be 

 partially or completely divided into triangles. 



The sub-genera Craseomys and Phaulomys indicate advanced 

 specialisation, but seem to be rather of "group" than of higher 

 value (see Anderson, Ann. and Mag. Nat. Hist., Oct. 1909, 

 317- and May 1905, 493). 



The genus contains a number of species, of circumpolar 

 distribution, from the shores of the Polar Sea (introduced in 

 Bering Island) through arctic, boreal, transitional, and some- 

 times subtropical zones, but absent from Greenland, the 

 islands of the Polar Sea, Newfoundland, Spitzbergen, Novaya 

 Zemlya, and Iceland. South it ranges in North America to 



This is variably developed in the different species ; it is present in about 25 

 per cent. ^iE.g. britannicus. 



