388 MURIDAZ—MICROTIN 4 
three infolds, and three inner salient angles with two infolds. 
In some species of Decerostonyx (D. torguatus and the extinct 
British D. gudielmz) m\ and m’* have, in addition to the parts 
enumerated above, a minute vestigial postero-internal salient. 
angle (Fig. 55, ~')—the last trace of what may once have been 
an important element in each tooth. 2 is essentially like #* in 
form, differing merely in the degree of reduction of its posterior 
portion ; the first five dentinal spaces are like those of m’; the 
fourth outer angle—the vestigial one in the anterior teeth—is 
here largely developed, as is also, on the inner side, the 
homologue of the minute postero- 
internal angles of #' and m” of D. 
torguatus and gulielmz ; these two 
triangles are incompletely sepa- 
rated from each other by the third 
inner fold, and together they form 
acrescentic posterior loop; briefly, 
this tooth has six orseven dentinal 
spaces, with four salient angles 
and three infolds on each side. 
The lower cheek-teeth of 
Dicrostonyx (as also of other 
Microtine) are in general like 
those of the upper jaw, but the 
Gt Die hi transverse loop is at the posterior 
Fic. 55.—CROWN VIEW oF RIGHT . 
Urren (A) AND Lower (8) Carex. instead of at the anterior end of 
TEETH OF Dicrostonyx torquatus. Drawn each tooth (Fig. 555 B). Mt, CONn- 
by M. A. C. Hinton (73 times life size). : : 
sists of a posterior transverse loop, 
preceded by seven substantially closed triangles, of which four, 
including the anterior, are internal and three external ; anterior 
of all is a loop of complex structure, into the composition of 
which some four or more originally distinct cusps enter ; 
occasionally one or two of the posterior of these may be com- 
pletely separated off from the more forward part of the loop 
by infolds in the usual manner, thus increasing the number 
of closed triangles to eight or nine; there are never fewer 
than nine dentinal spaces, five outer and six inner salient 
angles, and four outer and five inner infolds. The two 
posterior teeth, essentially like each other in form, are 

A B 
