III.] LITOPTERNA. 77 



Perissodactyla are concerned, was exceedingly poorly off for 

 ungulates during the Plistocene epoch. Nevertheless, the 

 country was very rich in hoofed mammals, not only during the 

 Pampean, but likewise during the Santa Crucian epoch, and in 

 this respect it was quite as peculiar as it is in its edentates. It is 

 not a little remarkable that three of the extinct subordinal groups 

 of the order which are confined to this realm exhibit a more or 

 less decided approximation, more especially in the structure of 

 their molar teeth, to the earlier northern Tertiary representatives 

 of the Perissodactyla. Hence it is probable that all the four 

 suborders in question have originated from a common ancestral 

 stock, although apparently before the perissodactyles were differen- 

 tiated from an earlier group known as the Condylarthra. The 

 date when the ancestors of the South American forms reached 

 their present home is, however, enveloped in mystery, and 

 although it is fairly certain that such ancestors had a northern 

 origin, yet it is highly improbable that they entered South America 

 from that direction. 



The first of the three extinct subordinal groups in question, 

 for which the name of Litopterna has been proposed, is the one 

 showing the nearest parallelism with the Perissodactyla, and is 

 typified by the genus Macrauchenia, of which the skeleton is 

 figured in the accompanying illustration. In this group the cheek- 

 teeth approximate in general structure to those of the well-known 

 European Oligocene genus Palaotherium, although in the typical 

 genus Macrauchenia they have been so modified as to render the 

 resemblance obscure. An essential characteristic of the upper molar 

 teeth (fig. 13) is to be found in the presence of two distinct lobes 

 to their outer walls. The toes of both fore and hind feet are 

 elongated, and constructed on the same general plan as those of 

 the Perissodactyla, never exceeding three in number on each foot, 

 and the middle one being symmetrical in itself. Moreover the 

 astragalus of the tarsus or heel-joint resembles the corresponding 

 bone of the latter group in having a deep pulley-like groove on its 

 upper surface for the articulation of the tibia, although inferiorly 

 it is unlike. The calcaneum, or heel-bone, on the other hand, 

 resembles that of the Artiodactyla in bearing a small facet for the 

 articulation of the fibula, or small bone of the leg. A more 



