390 ORTMANN — DISTRIBUTION OF DECAPODS [April 3, 



convenient study we may put them together here under the follow- 

 ing heads : 



i. Explanation of the absence of Potamocarcinince. in Brazil south 

 of the Amazonas river. 



2. Explanation of the absence of Parastacidce in Middle and 

 Northern South America. 



3. Explanation of the absence of Astacoides-Mke forms in Africa. 



4. Explanation of the absence of Potamobiidce in Central and 

 South America. 



5. Explanation of the absence of crayfishes in Central and South 

 Asia and on the Malaysian islands. 



We can dispose of the first point with ease ; indeed, we have in- 

 dicated above the chief cause of it. The Potamocarcinincz originally 

 (in the Upper Cretaceous) occupied only the region to the north 

 of the Amazonian interoceanic connection. In the Tertiary we 

 have a connection of northern South America with the Brazilian 

 mass (Archiplata) in the west (region of the Cordilleras), and we 

 see that the crabs availed themselves of this opportunity and spread 

 over the mountainous regions of Ecuador, Peru and Bolivia, possi- 

 bly here reaching a climatic southern boundary. The Amazonas 

 valley, however, remained sea during a much longer time and was 

 gradually and slowly filled by the deposits carried down from the 

 mountains. Thus, up to a comparatively recent time, it was im- 

 possible for the Potamocarciiiina to cross this depression. That 

 there now exists a possibility of crossing this old barrier is shown 

 by the existence of at least one species on the southern banks of the 

 mouth of the Amazonas near Pará. 



In regard to the second point — the absence of Parastacidm in the 

 main (tropical) part of Brazil — I can offer no explanation. They 

 must have immigrated into Southern Brazil early in Tertiary times, 

 and possibly we have to deal here with a climatic barrier. 



We may take together the other three points, since they appar- 

 ently are subject to identical causes. Looking at the original dis- 

 tribution of the crayfishes and crabs in Upper Cretaceous times, we 

 have to note the very remarkable fact (see p. 388, footnotes 1 

 and 3) that both together occupied the whole of Mesozonia, but 

 each different parts of it almost to their mutual exclusion. The 

 crayfishes seem to have existed in the Asiatic part of this conti- 



