LEMMING 



mouth of Thlewiaza River, where it was common and where 

 a series of about seventy, comprising adults and young of 

 both sexes, was secured August 4 to 8. A succession of low, 

 fiat, boulder-covered areas, which lay between the shore and 

 some shallow lagoons a few hundred yards inland, was occupied 

 by the animals. The ground was dry and well covered with 

 short, thick grass, through which their runways extended in 

 every direction. They burrowed extensively, sometimes 

 beneath boulders, but as often in the sides of tiny terraces or 

 from a flat surface. Their holes seemed to be connected in an 

 endless labyrinth. We captured several by suddenly over- 

 turning some of the boulders, but most were taken in traps set 

 in their well-trodden roads. They paid no attention to bait, 

 but were readily caught in runway traps. When taken alive 

 they showed considerable ferocity for animals of their size, 

 snarling and biting vigorously. The breeding season seemed 

 to be nearly over, but a few females contained from four to six 

 embryos. The teats are eight in number, four inguinal and 

 four pectoral." 



The migrations of Lemmings are one of the marvels of rodent 

 life. These great movements of Mice have been well known 

 in northern Europe from early times, and similar activity has 

 been noted in Arctic America. When conditions have been 

 unusually favorable for the Lemmings and the normal checks 

 on the annual increase have been unable to keep down the 

 numbers of the Mice, the Lemming population becomes too 

 vast for the available food supply. Great swarms of Lem- 

 mings start for new territory and move in vast waves of rodent 

 life which do not stop for rivers, lakes, or any obstacle of 

 topography. These hordes are followed by the natural 

 enemies of the Mice, Hawks, Owls, Foxes, etc., and through 

 the constant attacks of these and the inroads of disease and 

 accident, the multitudes gradually melt away. After such a 

 migration the numbers of Lemmings may be very low until 

 the recuperative powers of the species have brought the cycle 

 onto the upturn once more. 



Genus Dicrostonyx^ 



Dentition: Incisors, { ; Canines, ^; Premolars, §; Molars, f = 16. 

 ^ For a full revision of this genus see G. M. Allen, Bulletin Museum of 

 Comparative Zoology, Vol. LXII, pp. 509-540, 1919. 



401 



