EASEMAN, GEO GRAPHICAL DISTRIB UTION IN SO VTE AMERICA 79 



therefore retained more of the primitive characters than either the 

 Neniatognathi or the Cyprinidse. 



There have been sufficient connections between North America and 

 South America and between Eurasia and Africa to permit exchange of 

 fishes during past epochs. Besides, many fishes and other fresh-water 

 forms may have been able to migrate short distances along the coast or 

 from island to near-by islands. Hence no objections can be raised against 

 such migrations for want of land connections. 



Summary of the most Important Data which have been used to 

 support the View that South America and the Eastern Hemi- 

 sphere were Primitively connected. 



invertebrates 

 Crustacea 



Ortmann has stated that only one family of crabs found in northern 

 South America lends positive support to his view of Archhelenis, i. e., a 

 pre-Tertiary connection between Guiana and Africa. This family, Poto- 

 mocarcininse, are found only as far south as Guiana. They are found in 

 Central America. Why have they not immigrated into the Amazon Val- 

 ley and south? For all we know, they may have immigrated from the 

 northern hemisphere and have only reached Guiana. 



Their ancestral stock might have been drifted across the Atlantic by 

 the African-West Indian current or their ancestral stock, which was dis- 

 tributed, may easily have been distributed by the way of Europe and 

 North America. This is especially true in view of the fact that no iden- 

 tical forms are found in Africa and South America. What was this 

 paleotelic form which became distributed and gave rise to the different 

 genera of the eastern and western hemispheres, and where did it origi- 

 nate ? This family of crabs offers a splendid analogy to the present dis- 

 tribution of camels and tapirs which now live in the tropics, but which 

 were originally found in the northern hemisphere. Besides, the Mesozoic 

 forms of Crustacea are very imperfectly known, and this is especially true 

 of the crabs in question. 



Mollusca 



The writer found a bivalve, Diplodon, in the Iguassii Eiver above the 

 big falls, which in some form or other appears to date back to the Tri- 

 assic. This does not necessarily mean that Diplodon dates directly back 

 to the Trias, because this genus is widely distributed over the highlands, 

 but it does mean that it is a primitive form. In contrast to the above 



