GLIRES OF THE SANTA CRUZ BEDS. 461 



cisors are quite narrow and slender and have convex faces ; they are of 

 no great length, the lower one extending only to the line of m 5 . The 

 grinding teeth are prismatic and rootless. The upper premolar is not 

 molariform, as it is in Dolichotis, but is simpler than the molars, consist- 

 ing of a single, subtriangular column, with a well-marked groove upon 

 the anterior face. There is much variation, sometimes even in the same 

 individual, in the size of this tooth and in the depth of the groove. The 



FIG. 43. 



Grinding teeth of Caviidae. a. Right upper teeth, a' right lower teeth of Dolichotis magel- 

 lanica, x \. b. Right upper teeth, b' right lower teeth of Cavia cobaya, x f . 



first and second upper molars are each composed of two triangular prisms, 

 with apices directed inward and joined externally at the bases ; the faces 

 of the prisms are slightly convex, giving them a heart-like shape on the 

 masticating surface ; more or less cement is deposited in the valley be- 

 tween the prisms, at least in some of the species, otherwise these teeth are 

 quite as in Dolichotis. The third molar has a postero-external prolonga- 

 tion, which differs somewhat in size and shape in the various species, but 

 is always smaller than in the recent genus. 



In the crowns of the unworn teeth (see PI. LXVIII, fig. 27) may be 

 observed three broad and shallow valleys, which invade the crown from 

 the outer side, one in each prism and one in the space between the prisms, 

 which space is much broader than at a subsequent stage. Very early 

 these valleys become separated from the outer wall of the tooth and con- 

 verted into enamel lakes and, by the time that the animal is fully adult, 

 even the lakes have been entirely worn away (cf. figs. 236 and 27, PI. 

 LXVIII), while the permanent internal valley is so extended outward 

 that the prisms are connected only by a very narrow neck of enamel and 

 dentine. All the upper grinding teeth are so curved as to present the 

 grinding surfaces somewhat externally, but the curvature is not so strong 



