276 Transactions of the Society. 



The periodical drying up of the pools might be expected to 

 make them suitable only for species with a very short life-cycle, 

 or capable of surviving dry periods, and it is probably for that 

 reason that Cladocera, with exception of 'Macrothrix hirsuticomis, 

 are not usually found. The occurrence of the latter is difficult to 

 explain, inasmuch as it was only once found reproducing sexually; 

 it is possible that the species maintains itself in some parts of the 

 irrigation channels and is introduced afresh into the pools when 

 they are filled. 



Besides these palm-pools, I investigated three large ponds 

 which probably owed their existence to the superabundance of 

 water at that time, and are probably dry in summer. These 

 were : — 



1. Kasbah pond. — The old Turkish fort, or kasbah, at the 

 south of the oasis, is surrounded by a deep depression filled with 

 water forming a large pond divided into three parts by causeways. 

 This pond contained little vegetation, but a rich fauna, particularly 

 of Ostracods. 



List of species : — 



Daphnia magna Strauss. 



Ceriodaphnia dubia Eich. 



Macrothrix hirsuticomis Norm, and Brady. 



Alona elegans Kurz. 



Cyclops diaphanus Fisch. 



C. bisetosus Eehb. 



0. planus sp. n. 



Diaptomus chevreuxi De Guerne and Eich. 



Branchipus pisciformis Schaeff. 



Estheria sp. (larva?). 



2. Frog pond (plate VII.). — This name seems appropriate from 

 the immense number of frogs inhabiting the pond, which is situated 

 among the palms in the western part of the oasis. Here there 

 was a rich vegetation. 



EXPLANATION OF PLATE VIII. 



Pig. 1. — LepiduriLS hibbocki. Female, x 3. 



„ 2.— Ditto. Tail-plate, x 15. 



,, 3. — Streptocephalus bimaris sp. n. Male. Second antenna, x 37. 



,, 4.— Ditto. Female, 6th leg. x 26. 



„ 5. — Ditto. Female, tip of egg-sac. x 30. 



