8 PATAGONIAN EXPEDITIONS: PALEONTOLOGY. 



specialized and monodactyl Thoathermm as in the tridactyl and isodactyl 

 genera. When it is remembered that Thoatherimn greatly surpasses the 

 horses in completeness of digital reduction, the retention of so many 

 primitive characters becomes all the more remarkable and significant. 



The most important characteristics in which the Litopterna differ from the 

 Perissodactyla are points of agreement between the former and the Toxo- 

 dontia, most of which have tridactyl feet with mesaxonic symmetry. The 

 principal difference between the Toxodontia and the Litopterna is in the 

 structures of the auditory region, which, as Roth ('03) has pointed out, is 

 very remarkably specialized in the former, but normal in the latter. To 

 this question it will be necessary to return in the chapter dealing with the 

 Santa Cruz fauna as a whole, but I may so far anticipate that chapter as 

 to say that, in my judgment, despite this very striking and important dif- 

 ference, the Litopterna and Toxodontia "originated from a common an- 

 cestral stock," as Lydekker has expressed it. 



PROTEROTHERIID^. 



The two families into which the Santa Cruz Litopterna are divided are 

 sharply marked off from each other and represent two strongly divergent 

 lines of development. The Proterotheriidas are remarkable for the strik- 

 ing way in which they parallel and imitate the horses, especially in foot- 

 structure, and they even surpass that perissodactyl family in the complete- 

 ness with which the monodactyl foot is attained. 



The Santa Cruz representatives of the present family are all of small 

 or moderate size and most of them are slenderly built ; none are massive 

 or of great stature. 



All of the Santa Cruz genera have the same dental formula : I^, Ct, Pt, 

 Mf. The single upper incisor is enlarged into a small, more or less tri- 

 hedral and sharp-pointed tusk, growing from a persistent pulp ; the outer 

 lower incisor in all of the genera, except Thoathermm, is correspondingly 

 enlarged. The lower tusk bites behind the upper, which is therefore 

 worn on the posterior face. The median lower incisor is usually very 

 small and of little or no functional importance, having no upper tooth 

 opposed to it. The homologies of these incisors are not quite certain, 

 but there is much reason to believe that, as in the toxodonts, they are the 

 second upper and third lower of the original three (i- and i-3). The wide 

 separation between the two upper tusks makes it almost certain that at least 



