MOLLrSCA FROM THE SOUTHERN SUDAN. 255 



This species shows a certain amount of variation in form. Von Martens 

 (Malak. Bliitt. vol. xxi. 1873, p. 43) refers to a small specimen fomid by 

 Prof. Schweinfurth in the Bahr el-Ghazal as Unio sennaariensis, var. schwein- 

 furtJii, but as there is only one he hesitates about founding a new species on 

 it. Later, in ' Nov. Conch.' of PfeifEer, vol. iv. 1876, p. 140, pi. 132. figs. 3-5, 

 he describes it as Unio parreyssi, var. sclnoeinfurthi. Unfortunately the 

 shell is immature ; it resembles several in my collection, but it is impossible 

 to know whether it would have grown up in any way distinct from the 

 type in form, and the red colour of the nacre has no value as a specific 

 distinction. 



There are however two, if not three, distinct varieties. 



Var. OBLIQUA, nov. (PL 18. fig. 11.) 



Diagnosis. — Jickeli (Nova Acta Akad. d. Naturf. vol. xxxvii. 1874, p. 274) 

 mentions two shells found by Prof. Schweinfurth in the Bahr al-Ghazal which 

 he does not name. They are described as having the dorsal margin much 

 higher posteriorly and lower anteriorly. Thus the shells have an oblique 

 appearance. Also the dorsal margin is more or less angular at the anterior 

 end. 



Locality. — I took this variety at Tawila (two separate valves), Masran 

 Island (three), Melut (six). There are also two specimens from this last 

 locality intermediate between the variety and the type in shape. 



Dimensions. — The dimensions given by Jickeli are, long. 30, alt. 17*75, 

 crass. 11*5 mm. 



One example from Meltit is near this, while another is still laroer, 

 measuring long. 39'5, alt. at umbo 19, greatest alt. 24, crass. 16 mm. 



Var. ELONGATA, nov. (PL 18. figs. 9, 10.) 



Diaffnosis. — Shell more elongated, having the umbo nearer the anterior end. 

 The adult has the test thicker and more swollen and the teeth are generally 

 very strong and jagged. 



The tw^o valves from Tawila, which I have referred to the last variety, 

 resemble this in their great strength but are oblique in form. 



Locality and Dimensions. — The variety elongata occurred at Ad-Duwem 

 (two), Tawila (three), and Gebel Ahmad Agha (one). The last measures, 

 long. 42, alt. at umbo 21, greatest alt. 23'5, crass. 20 mm. The largest from 

 Tawila measures, long. 40"5, alt. at umbo 21'5, greatest alt. 23 5, crass. 

 18 mm. 



A small specimen found at Mogran and another at Khor Surkab near 

 Kerreri are shorter than usual and much swollen, also the posterior dorsal 

 margin is lower. The former measures, long. 31*5, alt. at umbo 19, greatest 

 alt. 19"5, crass. 15"5 mm. 



