MOLLUSCA FROM THE SOUTHERN SUDAN. 251 



Family Vagi NULiD^. 



Genus Veronicella, Blainville. 



Veronicella nilotica, Cocherell. 



A second specimen of this rare species was taken at Hillet al-Nuwer, 

 about which Mr. Robson gives further details in Appendix II., p. 2&%. 



Class PELECYPODA. 



Order EULAMELLIBRANCHIATA. 



Suborder SUBMYTILACEA. 



Family tEtheriid^. 



Genus jEtheria, Lamarck. 



^THERIA ELLIPTICA, Lamarck. 



JEtlieria elliptlca, Lamarck, 1807, Ann. Mus. Hist, Nat. Paris, vol. x. p. 401, pi. 29 and 

 pi. 30. fig. 1. 



Numerous specimens were taken at Ad-Duwem, Abba Island, Masran 

 Island, Gebel En, near Gebel Ahmad Agha, and at Mashra Zarafa Wood 

 Station. At several places clusters were observed formed of examples of both 

 the smooth, typical form, and the variety tuUfera, Sowerby*, adhering 

 together, this variety being generally the more abundant. The theory that 

 the latter is characteristic of calm and the former of more turbulent waters 

 can hardly be maintained. This species varies not only in ornamentation but 

 also in shape, and sometimes the shells form such masses as to be an im- 

 pediment to navigation. 



^L^tlieria elliptica has been recorded from the whole Nile basin, the Lakes 

 Victoria, Tanganyika, Dembea, Rudolf, and Tchad ; from the Blue Nile, 

 where I, as well as other collectors, have also taken it ; and from West 

 Africa ; a very similar form is stated to occur in the Congo. 



* Zool. Journ. vol. i. 1825, p. 523, pi. 19. 



22* 



