EDENTATA OF THE SANTA CRUZ BEDS. 351 



also the inner condyle has begun to be odontoid in form, but in quite a 

 different fashion and without any likeness to Mylodon. Another differ- 

 ence from the astragalus of the Megalony chides, seen in that referred to 

 Nematherium, is the division of the internal calcaneal facet into two dis- 

 tinct surfaces, which are almost at right angles to each other, while the 

 cuboid facet is smaller and the navicular facet is shallower. This astrag- 

 alus measures 35 mm. in length and 27 mm. in greatest width. 



The navicular resembles that of Hapalops, except that the inner border 

 is somewhat less produced proximally. In the fragment of the calcaneum 

 the only noteworthy feature is the presence of a second internal astragalar 

 facet, corresponding to the division of the inner calcaneal surface of the 

 astragalus. 



Relationships. The materials for the study of Nematherium are tan- 

 talizingly incomplete and entirely insufficient for any final determination 

 of its taxonomic position. However, so far as this material goes, it is all 

 in favor of the conclusion that this genus is a forerunner and quite pos- 

 sibly an actual ancestor of Mylodon, while its reference to the family of 

 the MylodontidcB appears not to be open to question. The resemblances 

 to Mylodon are especially marked in the scanty remnants of the limbs and 

 feet, which are just what we should expect to find in an ancestor of that 

 genus, while the anterior broadening of the palate and rostrum, though 

 not at all striking, is another significant resemblance. At the same time, 

 we find many points of similarity between Nematherium and Scelidothe- 

 riwn, especially in the characters of the teeth and skull, so that much 

 might be said in favor of regarding the Santa Cruz genus as nearly, if not 

 quite, representing the common ancestor of the two Pleistocene genera. 

 Even though the relationship between the earlier and the later genera 

 should prove not to be so simple and direct as is here assumed, these 

 Santa Cruz species would not be without importance, because they would 

 then represent what might be called arrested stages in the development of 

 the family, which, as Tournouet has shown ('03), was already in existence 

 in pre-Patagonian times. 



While the resemblance of Nematherium to Mylodon and the propriety 

 of referring the former to the Mylodontidce are sufficiently obvious, it is of 

 interest, on the other hand, to observe the many likenesses to the con- 

 temporary members of the Megalonychidcz and the Planopsidce. So many 

 and so close are these likenesses that, if the Santa Cruz genera of the 



