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Transactions of the Society. 



with a varying fauna. The following table shows the fauna of 

 four such pools : — 



LIST OF SPECIES. 



Malacostkaca. 



Potamon edule Latr. — The fresh-water crab is apparently widely 

 distributed in Algeria and Tunisia, but it was only seen by me in 

 Biskra and its neighbourhood. It seems to hide itself in cold 

 weather, and it was only in spring water which was somewhat 

 warmer than the air that it was noticed. Miss Eathbun has 

 recently (1904) separated the circum-Mediterranean form of Pota- 

 mon (P. fluviatilc Belon) into three species, P. edule Latr., P. 

 potamoios Oliv., and P. setiger Bathb. Tbe first of these, to which 

 my specimens belong, inhabits Italy, Greece, and Numidia, while 

 the second is found in Egypt, Balestine, Syria, Mesopotamia, and 

 Cyprus, the former being, therefore, the Western and the latter the 

 Eastern form. The separation of an Eastern from a Western Medi- 

 terranean form recalls the separation of Saxicola albicollis Viell. by 

 Whitaker into an Eastern form, S. amphileuca Ehr., and a Western 

 form, S. albicollis. The distribution of these two species agrees 

 almost exactly with that of Potamon fotamoios and P. edule 

 respectively. 



Paltemonetes varians Leach. — The first specimens of this species 

 found in Algeria were taken by Lord Walsingham, just before my 

 visit, in the conduit supplying the oasis of Oumach. I myself 

 found several more specimens in the outflow of one of the Sources 

 d'Oumach, but it seems to be far from common. It has been 

 recorded from several localities in Tunisia. 



Asellus aquaticus Linn. — A few specimens were found in pools 

 in the Jardin d'Essai at Algiers. 



Cirolana fontis Gurney. — A description of this interesting 

 species has already been given elsewhere.* Three specimens 



* Zool. Anzeig.,xxxii. (1908) p. 682. 



