Vol eee & 11.] The Fauna of the Jordan System. 467 
in its larval stages and at the breeding-season, can hardly 
be regarded as an aquatic animal, while the crab is known 
nean basin into two geographical sections ; one section occurring 
in North Africa west of the Delta and also in the northern 
has apparently been displaced by a more vigorous represen- 
tative of an allied genus (Caridina nilotica), a species most 
probably of Ethiopian origin, but one that has spread not only 
over practically the whole of the African continent, but also 
all over the Oriental Region and as far east as Celebes, without 
penetrating into Palestine. The tortoise is of course less com- 
_ pletely aquatic, but its distribution is similar to that of Atyaé- 
phyra. In the case of the reptile there seems to be no differ- 
ence in structure and colouration between specimens from North 
Africa, from Palestine or from Europe, but Bouvier,’ who was 
unaware that the prawn had been found in the Jordan, 
has recently distinguished two races of A. desmarestit, one 
(ortentalis) occurring in Syria, the other (occidentalis) in Europe 
and North Africa. The Jordanic form is probably identi- 
cal with the Syrian, but unfortunately my series from the Lake 
of Tiberias is deficient in males, and it is on the male characters 
that the races are mainly distinguished. ; 
The geological history of the Lower Nile is still obscure 
and the influence of large rivers in the zoogeographical history 
of aquatic animals, not only as paths of immigration but also as 
barriers, has not received the attention due to its importance as 
a factor in their distribution, It is any rate clear in the present 
instance that the Nile has been an obstruction rather than a hi gh- 
way so far as the freshwater fauna of the Mediterranean basin is 
concerned, and that it has acted in this capacity in two ways, 
firstly by separating those individuals that lived in North Africa 
before the river adopted its present course from those that 
lived immediately to the east and north of its delta, and 
1 Bull. Mus. d’ Hist. nat. (Paris) 1913, pp. 65-67. 
