RODENTIA— OCHOTONIDyE 



157 



29, No 1, b and c), and the inner border of the tooth is 

 indented by a wide notch or groove (a), which extends down 

 towards the root. In Prolagus the central upper premolar 

 ( Nos. 3, 4, pz) shows what is substantially the same arrange- 

 ment ; but the posterior upper premolar (Nos. 3, 4, p4) shows 

 a change ; in the latter the two crescentic enamel folds {b and c) 

 have been converted into enamel islets, and the inner notch a 

 has become a transverse fold, stretching half-way across the 



FlG. 29. — UPPER CHEEK TEETH OF DuplicUentata. I, Grinding surface and, la, anterior 

 view of a molar of Titanomys visenoiriensis (lower miocene) ; 2, the corresponding tooth in 

 7\ fontanessi (middle miocene) ; 3, 4, 5, grinding surfaces of pi, pi, and mi, in Prolagus 

 aningensis (middle miocene), P. sardus corsicanus (pleistocene), and Ochotona. spelora 

 (pleistocene), respectively ; 6, grinding surface of a worn milk molar of Oryctolagus 

 cuniadus (recent). All represent right teeth; in Figs. 1, 3-6, the left and right sides 

 are posterior and anterior respectively, the bottom and top are internal and external 

 respectively ; in la and 2 the right and left sides are internal and external respectively. 

 (Drawn by M. A. C. Hinton ; figs. 1, 2, 3, and 6 after Forsyth Major.) 



tooth in P. sardus (No. 4). In the earlier species, P. aningensis 

 (No. 3), the anterior upper molar (mi) has lost the outer cres- 

 centic fold c altogether in adult stages of wear, but the inner 

 one b remains as an islet ; the transverse fold a is still further 

 developed than in the posterior upper premolar (J>A). In the 

 pleistocene P. sardus, mi has lost both crescentic folds (No. 4), 

 and the transverse . fold a nearly reaches the outer border, as 

 in the living pikas, and in the hares and rabbits. Thus it is 

 shown that in the Ochotonidcs the anterior cheek-teeth are more 

 conservative, the posterior ones more progressive, and the same 

 holds good of the Leporidce, and also of many other rodents. 

 vol. 11. l 2 



