RODENTIA— OCHOTONID^ 



IS7 



29, No I, 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, /s) shows what is substantially the same arrange- 

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



f* /" 



Fig, 29,— Upper Cheek Teeth of Duplicidentata. i, Grinding surface and, Ifl, anterior 

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

 T, fontanessi (middle miocene) ; 3, 4, 5, grinding surfaces of /3,/4, and m\, in Prolagus 

 ceningensis (middle miocene), P. sardus corsicanus (pleistocene), and Ochotona speli^a 

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

 cuniculus (recent). All represent right teeth; in Figs. I, 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. I, 2, 3, and 6 after Forsyth Major.) 



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

 (No. 3), the anterior upper molar {m\) 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 (/4). In the 

 pleistocene/', sardus, m]_ 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 Ochotonidce 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. II. L 2 



