GLIRES OF THE SANTA CRUZ BEDS. 483 



above the level of the head, which is situated upon a longer and more 

 constricted neck. The shaft is heavy, broad and antero-posteriorly 

 compressed. 



The tibia (PI. LXIX, fig. 16) is merely an enlarged copy of that of 

 Eocardia, with somewhat straighter shaft. The fibula has a complete 

 though very slender shaft, which is more reduced than in E. excavata ; 

 the distal end is shaped as in that species. 



The astragalus is more asymmetrical than in Eocardia, owing to the 

 larger size of the external condyle, and the distal end is broader and 

 more depressed, an interesting approximation to Dolichotis. The only 

 other elements of the pes which have yet been found are the distal half 

 of the calcaneum and metatarsals II and III, and a few phalanges ; all of 

 these are so like the corresponding parts of Eocardia that description 

 would be superfluous. 



Relationships. --There can be little doubt, I think, that in Schistomys 

 we have the direct ancestor of Dolichotis. The only objection to this 

 conclusion that might be urged is the disparity in the length of the fore 

 and hind limbs in the fossil. We know of no reason, however, why in 

 the evolution of a phylum the hind limb should not first increase in size 

 and then, at a later stage, the fore limbs lengthen equally. This same 

 process has been observed in other phyla, notably that of the horses, in 

 which the Oligocene Mesohippus shows a similar disparity that, in subse- 

 quent genera, is removed by the elongation of the fore limb, and as already 

 mentioned, in the Prolagostomus J/iscaccia series. 



SCHISTOMYS ELLIPTICUS (Ameghino). 



(Plate LXIX, Figs. 11-16.) 



Eocardia elliptica Amegh. ; Rev. Argent, de Hist. Nat. T. I, 1891, p. 145. 

 Procardia elliptica Amegh. ; Ibid., p. 302. 

 Schistomys crassus Amegh. ; Ibid., p. 303. 



This species is distinguished by its larger size and generally more 

 advanced state of differentiation ; in size there is considerable variation 

 and it is evident that, as in certain existing -rodents, growth continued till 

 quite a late period in life. P 1 is large, nearly as large as m 1 and the 

 grinding surface of the upper teeth has a strong inclination downward 

 and forward. The cranium is high and vaulted, with upper contour decid- 



