14 Dr. G. B. Longstaft 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 N6, 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 
23° of latitude, and never more than forty miles distant, 
will here be treated as part of the same district. Khartiim 
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 Ladé and above, but with these exceptions the 
country is level. 
Between Kharttim 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) 
quadrangularis, Wallich [Nat. Ord. Ampelidae], a succulent 
jointed creeper, suggesting a Cactus. At the time of my 
