14 Dr. G. B. Longstaf! on 



Gebel. In practice, however, the name is usually held to 

 include the Bahr al-Gebel from Gondokoro or Rejaf, the 

 head of navigation [Lat. 4° 45' N.], down to Lake No, and 

 this is the sense in which the name is here used. Moreover 

 the Bahr al-Zarafa, which is practically a loop of the 

 Bahr al-Gebel, running more or less parallel to it for about 

 2|° of latitude, and never more than forty miles distant, 

 will here be treated as part of the same district. Khartum 

 again, though strictly speaking it stands upon the Blue 

 Nile, is included for reasons of convenience, being the 

 port of entry into the region. 



On the other hand the Bahr al-Ghazal, draining as it 

 does the large area between the Bahr al-Gebel and the 

 Congo basin, is not dealt with here. From what is known 

 of its fauna it would appear to comprise more insects 

 characteristic of Central and Southern Africa, than the 

 fauna we are here considering. 



The region thus defined lends itself to treatment as a 

 unit, both from the fact that it is served by the convenient 

 Government steamers and by the fact that it is throughout 

 fairly uniform in character. At Gebel Auli and at Gebel 

 En are small hills of igneous rock, while many similar hills 

 occur at Lado and above, but with these exceptions the 

 country is level. 



Between Khartum and Abba Island the country is for 

 the most part bare and open. A few Acacia (commonly 

 called Mimosa) trees or shrubs are here conspicuous by their 

 rarity. Another small tree or shrub commonly met with 

 on the desert is the " Nabbak," a species of Buckthorn, 

 Zizyphus mucronata, Wild [Nat. Ord. Rhamnaceae] — a 

 rather graceful tree whose white stems give it a Birch-like 

 character, but it is defended by a peculiarly malicious 

 scheme of thorns, which are arranged in pairs, one straight, 

 the other curved. The Acacias extend right up to Gondo- 

 koro. Among the shrubs especially interesting to the 

 Entomologist, are various Capers and other members of 

 the order Capparidaceae. They are closely associated 

 with Pierinae, whose larvae feed upon them. Another 

 shrub, especially common on and near Abba Island, is 

 Salvadora persica, Linn., also much frequented by Pierines; 

 it has numerous insignificant green flowers. A remarkable 

 plant with a wide range in the district is Vitis (Cissus) 

 quadrangidaris, Wallich [Nat. Ord. Atnpelidae], a succulent 

 jointed creeper, suggesting a Cactus. At the time of my 



