372 ORTMANN — DISTRIBUTION OF DECAPODS [April 3, 



Cretaceous — Africa, for a very long time, was isolated from the rest 

 of the Old World. After it had become disconnected from South 

 America, at the beginning of the Tertiary, it was absolutely 

 isolated, but soon during the course of the Tertiary it became 

 united with Asia and the new continent of Europe. The most impor- 

 tant stages in this process were that of the elevation of Western Asia, 

 in the Miocene, and the elevation of the northwestern parts of 

 Africa and southwestern parts of Europe at about the same time. 



This has the following bearing upon the origin of the distribution 

 of our freshwater Crustaceans: The African types of the subfamily 

 PotamonincE (chiefly the subgenus Potamonautes) could not reach 

 Europe before Miocene times, and, on the other hand, an immigra- 

 tion of the Asiatic types (subgenus Potamoii) into Africa (and 

 Europe) was also impossible before the Miocene and after the de- 

 struction of the Madagassian land-bridge in the earlier Eocene. 



Whether the alleged fossil species of Pota??ion from the Miocene 

 of Europe indicate this Miocene connection of Asia, Africa and 

 Europe remains doubtful. The lack of African types in the Medi- 

 terranean region of the present time, as well as the general absence 

 of a northerly and easterly advance of them (except in the Nile 

 valley, where the immigration no doubt belongs to a very recent 

 period), is opposed to the above assumption, and it is quite possible 

 that these fossil forms do not belong to relations of the Potamonince. 

 It seems that the desert zone of the Sahara already existed in 

 Miocene times, at least that it began to develop at the same time 

 that Western Asia became land, since just this process furnished 

 the conditions for the origin of an arid climate in North Africa and 

 West Asia. On the other hand, we see that a species of the sub- 

 genus Poiamon, belonging to the Indian fauna, advanced in a west- 

 erly direction across the new land areas formed in Miocene time, 

 and that it reached by this route Northern Africa (Egypt and 

 Algiers), But the distribution of this species (JPotamon fluviatile) 

 needs further explanation, since it is also found in certain parts of 

 Europe, and we shall discuss this question in the next chapter. 



IO. RELATIONS OF EUROPE TO ASIA. 



In discussing the distribution of Potamon fluviatile in Southern 

 Europe, just referred to, we are also to consider the presence of the 

 genus Potamobius in Europe, the area of which is separated from 

 that of the rest of the genus (in Northeast Asia and Northwest 



