EDENTATA OF THE SANTA CRUZ BEDS. 177 



of secondary sexual characters, but the analogy of the recent edentates 

 does not encourage us to expect any very notable sexual differences in 

 the fossil skeletons. Even less than usual is it possible to be consistent 

 in the employment of specific and generic criteria ; a character which in 

 one group is altogether variable and of no taxonomic value, may in another 

 be quite constant and prove of much assistance in distinguishing the 

 species. While the genera may, as a rule, be separated without great 

 difficulty, the only practicable course, in the present state of knowledge, 

 is to regard the species as vague and elastic groups, partly on account 

 of their variability and partly because so many have been established upon 

 fragmentary remains. The satisfactory taxonomic arrangement of the 

 Santa Cruz Gravigrada must await the gathering of much more extensive 

 collections. I have attempted in the following pages to furnish the reader 

 with material adequate to an independent judgment of these questions. 



Attention has already been called to the fact that the Gravigrada of the 

 Santa Cruz beds have great phylogenetic importance and that represen- 

 tatives, if not actual ancestors, of almost all the Pleistocene genera may 

 be found in this fauna. It should be observed, however, that the number 

 of Santa Cruz genera much exceeds that of the Pampean types, most of 

 the former having died out without successors. As examples of this may 

 be cited Ettckolceops, Analcimorphus, Pelecyodon, and many others. On 

 the other hand, several of the genera may well have been the direct ances- 

 tors of later forms. The Pleistocene Nothrotherntm might be sufficiently 

 described as Hapalops with the dental formula reduced to and with the 

 pterygoids curiously inflated. It is extremely probable that the North 

 American Megalonyx is descended from some of the Santa Cruz Megalony- 

 chidce, for there is the closest correspondence between them in all parts of 

 the skeleton ; the Pleistocene genus is very much larger and more mas- 

 sive and the skull has undergone considerable modification, but otherwise 

 there is singularly little change. Among the Santa Cruz representatives 

 of the family, the one most likely to prove the desired ancestor is Mega- 

 lonychotheriitm, which already displays the characteristic type of dentition, 

 but this genus is still too incompletely known for certainty in the matter. 



It is extremely unfortunate that the Planopsidce and Mylodontidce are 

 so rare and are represented in the collections by such incomplete speci- 

 mens. Under these circumstances it is impossible to decide questions of 

 phylogeny with any degree of confidence. So far, however, as the evi- 



