36 



MICROTIN^ 



seasonal change; with the approach of winter these two claws 

 grow to an extraordinary size and develop a peculiar supplementary- 

 ventral portion which sometimes surpasses the main part of the 

 claw in length ; but with the return of spring this ventral portion 

 is shed and the main claw is then worn down to normal length. 

 The bones of the fore-arm, particularly the ulna, are greatly 

 strengthened for the attachment of the powerful muscles which 

 move the fossorial hand. 



In this genus the incisors have become slender, straightened, 



Fig. 19. — Dicrostonyx grcenlandicus Traill. 



Skull and left mandibular ramus dissected to show the alveolar courses 

 of the teeth (enlarged). 



and more or less protruding; and they retain at the most but 

 a very feeble vestigial trace of an anterior groove. The cheek- 

 teeth, as in all other known lemmings, have become hypsodont, 

 persistently growing, broad-crowned, with their enamel differ- 

 entiated into thick and thin portions and reduced even to dis- 

 appearance on those portions of the periphery of the crown 

 where enamel is no longer of functional importance. The 

 intrusion of the alveolar capsules of m^ and ?«^ into the sphen- 

 orbital fissure has resulted in the lateral compression of the 

 presphenoid, which is reduced to a slender rod in most species. 

 The temporal muscles produce a salient ridge upon each side of 



