34 ARKIV FÖR ZOOLOGI. BAND 3. NiO 3. 



therium presents some characteristics in common^ with Ori/c- 

 ieropiis. This likeness makes Ameghino believe in »the South 

 American origin of Orycterojndce» . But he mentions a t the 

 same time differences which are of extremely greiit impor- 

 tance such as »la fusion de vértebras cervicales, la complica- 

 cion de las articulaciones de las lumbares etc». The presence 

 of these characteristics in Stegotherium makes t hat animal, ac- 

 cording to my opinion, impossible as a supposed ancestral 

 type from which Orycteropvs should ha ve sprung, because 

 the characters quoted represent so fundamental specialisations 

 that once aquired they could not have been entirely löst 

 without any traces. Especially, is this the case with the 

 complicated lunibar articulation which, no doubt, would have 

 been very useful for an animal with the habits of Orycteropus, 

 and thus retained if it ever had originated. It appears to 

 me that it would be easier to explain the similarity with 

 regard to the structure of the astragal us and calcaneus of 

 Stegotherium and Orycteropus, as well as other likenesses that 

 might exist between these two animals, as likenesses due to 

 a similar low position in the mammalian series and partly 

 due to analogy in consequence of similar habits. If we as- 

 sume that the ancestors of the South American Edentata 

 have been some kind of primitive mammals (ungulates) in 

 the beginning of the Tertiary epoch the general structure of 

 these animals ought to have been very similar to that of the 

 primitive ungulates from which Orycteropus originated. If to 

 this theory about a similar origin is added the analogy of 

 similar habits of life these two items will explain, as I think, 

 quite satisfactorily the resemblance that exists between Oryc- 

 teropus and Stegotherium, primitive characteristics as a retai- 

 ned inheritance from low-standing mammalian ancestors, and 

 specialised features, if being similar, as products of paral- 

 lelism. But even when all this has been accounted for there 

 remains so many differences that a direct derivation of 

 Orycteropus from South American Edentata appears impos- 

 sible. 



The hypothesis forward ed above will help to explain not 

 only the resemblance in the structure of the tarsus of Stego- 

 theriidce and Orycteropus described by Ameghino, but also 



^ »algunos cai'actei'es en comiin con Orycteropus ^> (1. c. p. 88). 



