SCOTT : LITOPTERNA OF THE SANTA CRUZ BEDS. 5 



the arrangement of the bones in the carpus and tarsus is much more 

 primitive than in even the most ancient and primitive known perisso- 

 dactyls. In the carpus the arrangement is almost serial, except that the 

 scaphoid extends over upon the magnum, though the lunar is widely 

 separated from the unciform. The calcaneum bears a large facet for the 

 fibula and the astragalus has a depressed-convex head, which rests only 

 upon the navicular and has no contact with the cuboid. The tarsus is 

 thus not at all perissodactyl in character, but is more like that of the 

 Toxodonta, Typotheria and other South American ungulate groups. 

 The mode of digital reduction is that which Kowalevsky has called "in- 

 adaptive," and even in the monodactyl Thoatherimn, the articulations of 

 the carpals and tarsals with the metapodials show but little modification 

 of the plan characteristic of a pentadactyl foot. 



SYSTEMATIC POSITION OF THE LITOPTERNA. 



That the Litopterna have a strong resemblance to the Perissodactyla, it 

 needs no argument to prove, but is obvious at the first glance. The inter- 

 pretation of this resemblance in terms of genetic relationship is a very 

 different and by no means easy problem. It is not surprising that students 

 of the Litopterna should have reached opposite conclusions regarding the 

 systematic position of the group, for this is merely another case of the oft 

 recurring problem, as to how far certain resemblances are offset by differ- 

 ences of structure. The answer to this question is largely conditioned by 

 the opinion which is held concerning the mode in which the evolutionary 

 process acts, and how generally similar structures are acquired in unrelated, 

 or distantly related groups. Even though such parallelism of development 

 be admitted to the fullest extent, great difficulties arise in the application 

 of the principle to concrete cases and in distinguishing between those 

 resemblances which are the evidences of genetic relationship and those which 

 have been independently acquired. No better illustration of this general 

 problem could be desired than that afforded by the Litopterna. 



It would seem that the general opinion is that the Litopterna are nearly 

 related to the Perissodactyla. Ameghino, who first distinguished and named 

 the group, has all along maintained its perissodactyl relationship, and, in 

 his later publications, has not employed the term Litopterna, but has simply 

 referred the families to the Perissodactyla ('04, 52). This is the more 

 remarkable, since he now separates the horses from the Perissodactyla as 



