HARES AND RABBITS 211 



whereas in the Ungulates the tendency is rather in the opposite 

 direction, for the lower incisors to remain and the upper to 

 dwindle or disappear. 



The Rodents are an extremely ancient group, dating back 

 in time to the Eocene, or first period of the Tertiary Epoch. 

 Their forms at the present day are sharply divided into two sub- 

 orders, the Duplicidentata^ or rodents (such as hares) with at 

 least two pairs of incisors in the upper jaw, and the Simplicidentata 

 (such as squirrels, porcupines, mice), with only one pair of 

 incisors in the upper jaw. 



The Duplicidentata are divided at the present day into two 

 distinct families, one containing the pikas and the other the 

 hares, rabbits, and a remarkable form, Romerolagus^ which is in 

 some respects aberrant, and in others a link between the pikas 

 and the hares. 



Family: LAGOMYID^. THE PIKAS 

 These Rodents, represented by the genus Lagomys^ are smaller 

 than the hares and rabbits. They have short ears, no external 

 tail, the fore and hind limbs are about equal in length, the 

 incisor teeth are two on each side in the upper jaw, and there 

 are premolars, either two on each side or only one. The present 

 distribution of the Lagomyida is confined to Eastern Europe 

 (Russia), Northern and Central Asia (from Persia to Kamshatka) 

 and the western territories of North America (Rocky Mountains). 

 In the Pleistocene period, however, pikas of the existing Siberian 

 species {Lagomys alpinus) were found in the southern half of 

 England in common with so many other beasts which inhabit 

 Central Asia at the present day — gazelles, saigas, jerboas. The 

 pika lingered on in Sardinia almost to the Historical Epoch, but 

 apparently became extinct in Britain at a more remote period. 



Family : LEPORID^. HARES AND RABBITS 

 It would seem as though in our review of the origin of 

 British Mammals we were for once to leave India as the starting- 

 point of an important group and turn to North America. 



