i 5 6 RODENTIA— OCHOTONIDiE 



teeth, is of considerable interest, and throws so much light on 

 the teeth of other rodents that it may be described in some detail. 

 In common with those of other primitive mammals the cheek- 

 teeth of all the earliest Duplicidentata were short-crowned, or 

 brachyodont, and provided with long and completely closed 

 roots in the adults. As in other mammals, so (and nowhere 

 more strikingly) in the Rodentia — one of the principal directions 

 of dental advance has been that leading to hypsodonty, or long 

 crowns, and persistent growth. In the lower miocene period 

 the OchotonidcE had already made substantial progress along 

 this line. The anterior view of an upper molar of Titanomys 

 visenoviensis is given in Fig. 29, No. \a. The outer part of 

 the crown is very low, or brachyodont, and it is supported on 

 two small centrally placed and completely closed roots, of which 

 one is seen in the drawing. The inner part is supported on a 

 single large, but not quite closed root, and is greatly deepened 

 by an extension of its enamel covering over part of the surface 

 of the fang. The next stage is seen in the middle miocene 

 T. fontanessi, wherein the brachyodont external part of the 

 crown (No. 2) has atrophied ; the small fangs are no longer 

 central but external, and they remain longer unclosed. At the 

 same time the inner part of the crown has become still higher 

 or deeper, and the enamel has invested the whole of the inner 

 face of the enlarged and now constantly open root. Eventually 

 the small external roots are completely absorbed by the large 

 inner one ; the large internal root is itself transformed into part 

 of the molar crown by the encroachment of the enamel sheet, 

 and in the end we have the prismatic, persistently growing 

 teeth of the living pikas, and the hares and rabbits. To-day 

 the ancient rooted and brachyodont character is, among Dupli- 

 cidentata, alone met with in their milk-molars, these being 

 always more conservative than the permanent ones. 



Another interesting set of changes call for a little notice, 

 and is illustrated by Fig. 29. The cheek-teeth of Ochotonidce 

 and Leporidce originally possessed a longitudinal pattern of 

 enamel folds ; this has been superseded by a transverse 

 arrangement. An upper molar of Titanomys shows on its 

 worn surface two crescentic enamel folds each filled with cement, 

 and opening on the antero-external corner of the tooth (Fig. 



