HISTORY OF THE fLITOPTERNA 499 



to it in the drawing ; upon such a point there can be no 

 certainty. 



In the older formations preceding the Santa Cruz, the 

 fmacrauchenids are known only from fragmentary material, 

 though something of their history may be made out even from 

 these fragments. fProtheosodon, of the Deseado stage, was 

 considerably smaller than the Santa Cruz genus and had more 

 primitive upper molars, in that the internal cusps and inter- 

 mediate cuspules were isolated and conical, not forming trans- 

 verse crests. Still smaller were the several genera {\Lamh- 

 daconus, etc.) related to the fmacrauchenids found in the Casa 

 Mayor Eocene, which have been referred, perhaps correctly, 

 to the fCondylarthra. In these the formation of the external 

 wall of the almost bunodont upper molars was in progress, by 

 the fore-and-aft extension and transverse thinning of the ex- 

 ternal cusps ; the internal pair of cusps and the cuspules were 

 separate and conical. With much confidence, it may be in- 

 ferred that in these little animals the skull was normal, the 

 nasal bones were long and that the feet were five-toed, but 

 demonstration is lacking. 



The second family of the fLitopterna, the fProterotheriidae, 

 were remarkable for their many deceptive resemblances to the 

 horses. Even though those who contend that the fLitopterna 

 should be included in the Perissodactyla should prove to be 

 in the right, there can be no doubt that the fproterotheres 

 were not closely related to the horses, but formed a most strik- 

 ing illustration of the independent acquisition of similar char- 

 acters through parallel or convergent development. The 

 family was not represented in the Pleistocene, having died out 

 before that epoch, and the latest known members of it lived 

 in the upper Pliocene of Monte Hermoso. In the still older 

 Parana formation more numerous and varied forms occurred, 

 but only from the Santa Cruz have materials been obtained of 

 sufficient completeness to furnish a full account of the struc- 



