RODENTIA— OCHOTONID.E 155 



it not improbably had its origin. At a later date it spread 

 eastwards, and appears to have first found its way to America 

 by way of Asia in the pleistocene period. 



The Pikas are about the size of guinea-pigs, and are chiefly 

 found in mountains, where they live in holes in rocks. The 

 living forms differ from the hares in having all four limbs of 

 about equal length, in their short ears, and in the absence of a 

 tail. In the generally depressed skull the contracted frontals 

 are without postorbital processes ; the malar bones are drawn out 

 in very long processes behind the zygomatic processes of the 

 squamosals, and more than half of the bony palate is formed 

 by the palatine bones. The clavicles are complete. 



The Pikas differ from almost all Leporidce in having only 

 five upper cheek-teeth instead of six ; in the mandible there are 

 five or four cheek-teeth. The anterior three of these teeth in 

 the upper, the anterior two in the lower jaw always replace 

 deciduous predecessors, and are therefore premolars. In this 

 the Ochotonidce agree with the Leporidce ; and the tooth that is 

 constantly missing is the last upper molar (m$), but in some 

 forms the last lower molar {m$) disappears as well. The dental 

 formula of the Ochotonidcz is therefore : — 



.2-2 0-0 s _ 3 2-2 , 



1 , c , pm - — - , m = 24 or 20 ; 



1 — 1 0-0 2-2 2-2 or 3-3 



that of the Leporidce normally : — 



.2-2 O-O X — 7. 7. - T. 



i , c , pm - — -, m - — - = 28. 



1-1 0-0 2-2 3~3 



The suppression of m$ is a very ancient feature of the 

 Ochotonidce, since it is already seen in the oldest known form, • 

 viz., Titanomys visenoviensis of von Meyer from the lower 

 miocene. In a middle miocene successor, T. fontanessi of 

 Deperet, the last lower molar (wj) also disappears in adults, 

 and this tooth is entirely suppressed in Prolagus (middle 

 miocene to pleistocene). The genera Lagopsis (middle miocene) 

 and Ochotona (pleistocene and recent), although more special- 

 ised than Prolagus as regards the structure of the teeth, are 

 more primitive in that they have retained in$ ; this tooth being, 

 of course, as elsewhere in Duplicidentata, much reduced. 



The dental evolution, and especially that of the upper cheek- 



