Vol. XI, Nos. 10 & 11.] The Fauna of the Jordan System. 469 
[N.S.] 
correlated, as I have pointed out above, with the fact that 
thin-shelled species, some of which occur in neighbouring parts 
of the Jordan-system, cannot survive in the water of the 
lake. 
e fauna of the lake differs very considerably in its 
essential characters from that of the much larger bodies of water 
the existence of that lake was probably short and the state- 
ment frequently made by geologists that shells found in raised 
beaches near Tiberias which marked former water-levels, are 
identical with those that now occur in the lake is in most cases 
correct only so far as genera are concerned; in some 
incorrect. even to this extent. Melania tuberculata certainly is 
been made and Blanckenhorn ! cites among the identical species 
representatives of thin-shelled genera (Ancylus and Limnaea) 
that do not occur, or occur only in great scarcity, at present in 
the Lake of Tiberias. 
All these facts seem to me to point to their having been 
a period in the history of the Jordan system at which the 
fauna of the Jordan Lake, or rather of the relics that remained 
of it, was subjected to great hardships. Under these hardships 
& considerable number of species perished. A few, however, 
crawling over land from pool to pool or from stream to stream. 
Probably some of the animals that survived underwent a 
period of hardship and thus became endemic species, while 
some of the immigrants also unde 
their arrival. 
e easiest way to account for the occurrence of such 
a period seems to me to be the suggested existence of a tempo- 
rary obstruction of the outflow of the Lake of Tiberias—and 
probably also of other parts of the Jordan system—that 
caused a considerable rise in salinity. When this obstruction 
| Naturwiss. Stud. Toten Meer und Jordantal, p. 339 (Berlin: 1912). 
