DICROSTONYX 141 



of the old ones. In the autumn moult the whitening usually 

 starts low down on the flanks and extends gradually upwards 

 and forwards to the crown of the head, whereas in the spring 

 moult the coloured coat of summer first appears upon the head 

 and shoulders and gradually extends backwards and downwards. 

 The process is nevertheless subject to a great deal of individual 

 variation.^ 



Skull (Fig. 69) rather strongly built, moderately broad and 

 but little depressed. Rostrum, in correlation with the rather 

 weak upper incisors, light and slender, the anterior palatal 

 foramina rather large, the diastema and nasals rather long. 

 Nasals extending backwards about as far as the ascending 

 branches of the premaxillaries, ending at a point usually a httle 

 in front of a line connecting the anterior margins of the orbits. 

 Zygomatic arches given off squarely from the sides of the rostrum, 

 the greatest zygomatic breadth faUing in the anterior or maxillary 

 parts of the arches and amounting to between 62 and 70% of 

 the condylo-basal length. Upper border of jugal raised into a 

 moderately high convex crest for the insertion of the temporal 

 fascia. Temporal ridges salient throughout; in the moderately 

 constricted interorbital region they are usually persistently 

 separated by a longitudinal sulcus, which becomes, however, 

 narrower and deeper with age until, in some exceptional indi- 

 viduals, the groove may be interrupted at one point by the ridges 

 coming into actual contact with each other. Posteriorly the 

 temporal ridges traverse the parietals and squamosals, but their 

 course is slightly below the level of the lateral borders of the 

 interparietal. Anteriorly the parietals are widely and shallowly 

 emarginated by the intertemporal portion of the coronal suture. 



* An interesting experiment made by Ross,* in arctic seas, nearly a 

 century ago is worthy of notice. He kept an individual in its summer 

 coat alive in his cabin through the winter months, and on February 1 it 

 still retained the summer pelage. On that date Ross removed the animal 

 from the cabin to the deck and there exposed it to the rigours of the arctic 

 winter. Within a week, when the experiment was terminated by the death 

 of the captive, the coat turned wholly white except in a relatively small 

 area over the spine. By clipping og the white hair-tips Ross found that 

 he could restore to the animal the appearance it possessed while in summer 

 pelage, and he therefore thought that the change had taken place by a 

 bleaching of the tips of the hairs. The true interpretation is doubtless, 

 however, that given by MiddendorfE,t viz., that the hairs of the winter 

 coat were all present though concealed under the tips of the summer hairs 

 while the animal remained in the cabin, the unnatural warmth of that place 

 retarding their growth and so delaying the shedding of the old hairs ; that 

 the sudden exposure accelerated the growth of the new hairs which rapidly 

 attained their full length and concealed the far shorter hairs of the old 

 coat; and that finally sufficient time did not elapse between the exposure 

 to cold and the death of the animal to permit of the old coat being cast, 

 and therefore Ross was able to bring that old coat once more to the surface 

 by cutting off the ends of the newer and longer hairs. 



* Ross, App. Narr. Second Voyage, 1835, Nat. Hist., p. xiii. 

 t MiDDENDORFF, Sibirische Reise, 2, Th. 2, p. 91, 1853. 



