MOLLUSCA FKOM THE SOUTHERN SUDAN. 257 



type in the Bahr al-Abyad. It was found at Mogran (fifteen), Ad-Duwem 

 (seventeen), on Abba Island, both opposite Fachi Shoya (three), and at 

 Tawila (thirteen), Kosti (two), Hillet Abbas (five, and five separate valves), 

 Masran Island (one), Grebel En (six), north of Gebel Ahmad Agha (two 

 very small), south of Melut (three), Lul (one), Taufikiyfi (one valve). 

 Total sixty-eight. Reported also from the White Nile by M. Pallary. My 

 largest specimen occurred a little to the south of Melut; it measures, long. 

 44, alt. 17'5, crass. 16*5 mm. 



Genus MuTELA, Scopoli. 



MUTELA NILOTICA, SoW. 



Iridina nilotica, Sowerby, ]824, Zool. Journ. vol, i. p. 53, pi. 2. fig. 1. 

 Iridina nilotica, Cailliaud, 1827, ' Voyage a Meroe/ vol. iv. descr. p. 262 ; 1823, Atlas, 

 vol. ii. pi. 60. fig. 11. 



A single valve of a specimen of this species was found by Dr. Longstaff 

 near Gordon^s Tree, Khartum. It is not so large as Sowerby^s type, but 

 agrees with it in form, the dorsal and ventral margins being nearly parallel. 

 It measures, long. 9(), alt. at umbo 32, alt, post. 36 mm. Sowerby states that 

 his specimen M'as given to him by M. Cailliaud, and that it came from 

 Sennaar. A shell in the British Museum (Nat. Hist.) is probably the holo- 

 type; it agrees with the figure in shape, and has the adventitious marks in 

 the nacre, which are represented in the drawing, only reversed as would be 

 the case if they were copied from the reflection in a mirror. There is no 

 certain record of the locality of this individual. It measures, long. 144, 

 alt. .54 mm. 



The name nilotica appears to have been given by Ferussac in MS., but 

 Sowerby was the first to describe it, thougli the shell had been })reviously 

 figured by Cailliaud. 



This species greatly resembles the West African form, M. exotica. Lam. 

 [elonqata, Sow.), but it is distinguished by only having traces of crenulations 

 along the hinge-line, while they are very strongly developed in M. exotica. 



Cailliaud's figured specimen came from Joseph's Canal, Lower Egypt. 

 The species also occurs in Upper Egypt, the Blue and White Niles, and 

 Albert Nyanza. 



Var, ANGUSTATA, Soioerbr/, sp. 



Iridina angustata, Sowerby, 1868, in Reeve's ' Conch. Icon.' vol. xvi. pi. 2. figs. 4 & 6. 



Iridina nilotica, Savigny, 1813, ' Descr. Egypte,' pi. 7. fig. 2. 



Iridina nilotica, Audouin, 1826, ' Explic. Somm. Planches Moll. Egypte,' p, 46. 



Twenty specimens were taken: near Mogran (seven), at Tawila (one), 

 Gebel En (one), Gebel Ahmad Agha (four), Masran Island (five), and a 

 single valve, south of Melut (two). This form shows a certain amount of 

 variation. The shells found near Mogran come nearest to the holotype of 



