462 PATAGONIAN EXPEDITIONS I PAL/EONTOLOGY. 



as in the recent Cavia and Dolichotis, and the anterior convergence of the 

 two upper dental series is much less than in them. The fore-and-aft 

 plane of the grinding surfaces is nearly horizontal, without the inclination 

 downward and forward which is so conspicuous in Dolichotis. 



The lower premolar is subject to much individual variation, as well as 

 to considerable specific differences ; I have observed several instances in 

 which notable differences may be seen in the form of this tooth on the 

 two sides of the same jaw. The crown is molariform and consists of two 

 prisms, the anterior one of which is usually smaller than the posterior 

 and may be of either elliptical or triangular cross section, while its anterior 

 face may or may not have a vertical groove. When present, this groove 

 varies greatly in depth, even in the same individual and in some species 

 it becomes so deep as to divide the anterior prism into two, but every 

 stage of the transition to this extreme may be observed. The lower 

 molars consist each of two triangular prisms, with the valley between them 

 opening outward, and the teeth are curved, so as to present the grinding 

 surfaces somewhat inward and thus oppose those of the upper teeth. The 

 unworn lower teeth have internal shallow valleys, corresponding in num- 

 ber and position to the external valleys of the upper teeth. In both upper 



FIG. 44. 



Eocardia sp., teeth of young individuals, lower right side, external and crown views, x f . 

 a. Dp in very young stage, b. Dp ? , m T and j, later stage, A. M. N. H. 



and lower molars the narrow edges of the prisms are usually set off by 

 exceedingly fine grooves to form obscure pillars ; this feature is, however, 

 extremely variable. 



The milk premolars, which remain in function until after the eruption 

 of the last molar, differ from all of the permanent teeth in having roots. 

 Otherwise, dp- has the pattern of the molars (PI. LXVIII, fig. 27) and is 

 composed of two prisms, separated by a deep internal valley, with three 

 enamel folds from the outer side of the crown, which soon become isolated 



