6i6 



PATAGONIAN EXPEDITIONS : ZOOLOGY. 



and Asia. The occurrence of both families in the same waters is evidently 

 due to migrations, which ensued after the families had become differ- 

 entiated. The Mittelidce attain their northernmost point in the Panuco 

 River, in northeastern Mexico, where the family is represented by one 

 species of the South American genus Anodontites. 



Simpson and Germain refer a few African forms to the South American 

 genus Diplodon, and Germain has called attention to the similarity of the 

 African Pseudavicula to the South American Prisodon or Hyria. If the 

 resemblance of the shells proves to be supported by the soft anatomy, 

 then this family at least will be evidence of an eastward migration in 

 Archhelenis. At present the evidence is insufficient. The possibility of 

 convergence in shell-form must be taken into account. Most if not all 

 African Unionidce, like the African Cyclostomatidce, Zonitidce, etc., are 

 clearly of Oriental derivation. These are northern Caenogaeic families 

 which have invaded Africa during the Tertiary. 



The maps following illustrate the distribution of part of the Archhelenic 



groups. 



FIG. 20. 



Distribution of the land-snail family Achatinidce. The numerals represent the number of 

 genera in each area. 



The autochthonous families, together with the Archhelenic group, make 

 up the mollusk fauna of the Brazilian plateau ; but the more isolated and 

 peculiar, presumably older, genera have outlying forms in the Guiana- 

 Colombian center, indicating a former unity of the northern and southern 



