30± ORTMANN"— DISTRIBUTION OF DECAPODS [April 3, 



(Lower Egypt and Algiers) points, like the whole of the Mediter- 

 ranean region, to India, from which locality the species present there, 

 P. fiuviatile (Latr.), has apparently migrated in an east-westerly 

 direction over Persia, Mesopotamia and Syria. P. fiuviatile has 

 been actually recorded from western India ; at any rate the most 

 closely allied species to this one are found in India and China. 



Other remarkable facts in the distribution of this subfamily may 

 be summed up thus : 



i. The Asiatic as well as the African part of the range is occu- 

 pied by the subgenus Potamonautes. It is impossible to say which 

 was the original home of Potamonautes, but this much is evident, 

 that it must have been present in both parts at a comparatively 

 early time, it being probably older than Potamon sens, strict. In 

 Africa Potamonautes attained its highest development, being the 

 prevailing type there and showing great variety. 



2. Madagascar, while belonging distinctly to Africa in its fauna, 

 possesses some rather peculiar types. 



3. The subgenus Potamon originated in Asia, apparently at a time 

 when there was no connection any more with tropical Africa or 

 Madagascar. The immigration of Potamon into the Mediterranean 

 countries, across Persia, etc., is probably a comparatively recent 

 one, since the route of immigration is easily traced and occupied 

 by one single species. 



4. The Malaysian and Philippine Islands, Japan and North Austra- 

 lia possess in Geothelphusa a very peculiar group. This distribution 

 of Geothelphusa does not correspond to that of Parathelphusa, 

 Potamonautes and Potamon sens, strict., which are also found in the 

 Malaysian Islands. Potamonautes and Parathelphusa are similar in 

 this respect, possessing on the Sunda Islands only scattered stations 

 (as far as New Guinea), which by their discontinuity express an 

 ancient condition. Potamon points directly to an Indian origin, 

 extending only to Sumatra, Java and the Philippines, but going not 

 any farther to the east. 



5. The position of Parathelphusa is hard to understand. If it 

 is really absent in Africa, as we believe, its distribution in Asia is 

 rather eastern than western, being chiefly found in Farther India. 

 Its extension over the Sunda Islands to New Guinea points to old 

 conditions. Since the morphological relations of Parathelphusa to 

 the rest of the subfamily are not well understood, it is better to 

 exclude it from our further consideration. 



