On Fresh-water Crustacea. By Robert Gurney. 275 



being available for the oasis itself. An additional canal, the 

 " Canal des Crues," was dug in 1878 to catch and utilise the flood- 

 water of the river, but its supply is necessarily uncertain and 

 intermittent. The main canal sends branches all over the oasis, 

 and these branches, or seguias, subdivide till every parcel of land 

 is supplied with water, the surplus finding its way into ponds and 

 finally into the desert at the south of the oasis. The watering of 

 the palms is effected by means of pools dug at the foot of each 

 tree, or group of two or three. These pools are filled at varying 

 intervals according to the season, in summer (June to September) 

 about every seven days, in winter about every sixty-one days, and 

 in spring (February to May) every twenty-five days. The amount 

 of water allowed per palm for each watering is about 3 cubic 

 metres, the annual supply working out at about 72 cubic metres 

 per palm. These pools are usually quite small, about 4 metres 

 square in extent, but vary in size according as they supply one 

 palm or more than one ; they always have a muddy bottom and 

 rarely contain any vegetation, one or two being seen with a little 

 Xitella. The typical association of species found in them consists 

 of Branchipus pisciformis, Macrothrix hirsuticornis, Cyclops bicus- 

 pidattos var. lubbocki, C. cliaphanas, Cypris virens and Herpeto- 

 cypris reptans, but other species are also found occasionally. 

 In seventeen pools examined in Biskra sixteen species were found, 

 of which the following is a list, with their frequency expressed as 

 occurrences per 100 collections. The fauna of these pools appears 

 to be the same as in the neighbouring oases of Oumach and Sidi 

 Okbar. 



Table showing the Crustacean Fauna of Seventeen Palm Ierigati n 

 Pools in Biskra, and the Relative Frequency op the Occurrence 

 of the different species. 



u 2 



