MICROTINA 385 
substances, the cheek-teeth and masticatory muscles are 
exceptionally powerful; the skull is massively constructed, and 
several of its paired bones, such as the maxille, palatines, and 
frontals, which usually remain free in other J/u77zde, are fused 
together at an early stage in the existence of the individual 
(see Winge, Vzdensk, Med. Nat. For. K7ob., 1881, 37-50 (1882) ; 
Gronlands Pattedyr, 1902, 358; Danmark’s Pattedyr, 51, 68-79). 
The cheek-teeth are hypsodont and often endowed with the 
power of persistent growth. Their evolution, which is analogous 
to that of the pikas (see above, pp. 155-158), has been worked 
out by Hinton, to whom I am indebted for a résumé of his, at 
present unpublished, results. Although the actual ancestors of 
the Wicrotine have not yet been discovered, there can be no 
doubt that their cheek-teeth were of a brachyodont, tubercular 
type; the apical or first-formed and most conservative parts of 
these teeth in recent forms still retain such a structure, and in 
the more primitive genera, such as Dzcrostonyx, remains of three 
longitudinal rows of tubercles are present in both upper and 
lower series when unworn, showing that the group has descended 
from a stock with murine rather than cricetine cheek-teeth. 
Such brachyodont teeth were adapted for crushing and grind- 
ing; they have gradually been converted into a most perfect 
apparatus for slicing and shearing. The first step in the 
process has been the atrophy, by fusion or blending, of some of 
the less useful primitive tubercles. The cusps have thus been 
arranged in an inner and outer alternating series of triangular 
form, the evolution of which can be readily appreciated by 
examining the changes in pattern shown by the molars of 
Cricetene in different stages of wear (Figs. 52 and 54). 
The transition from a soft and succulent to a tough and dry 
diet has been gradual. It has been accompanied by steadily 
increasing wear of the crowns of the cheek-teeth, by increased 
nutrition of these organs, and by prolongation of their period 
of growth. Since during the earlier stages growth exceeds 
attrition, the cheek-teeth have become hypsodont; in later 
stages growth and wear are equal, and this condition persists 
until death in the higher genera of the group, as in JWzcrotus ; 
in lower forms, as Avotomys, a time comes when the enamel 
organs fail, and thereafter growth of the crowns ceases, the 
