PRESIDENT'S address. 
465 
Crustacea known to us, either living or extinct. While the 
genera are in some cases restricted in distribution, the majority 
are widely spread, and one species — for example, Cyclestheria 
hislopi — is found in India, Australia, Africa, and South 
America. A form such as this, which is structurally of the 
greatest interest, is comparable in its immutability to the classic 
example of Lingula. 
Of course, the question of the first origin of the fresh water 
fauna is the purest speculation, without any possible foundation 
of knowledge, and it must be remembered that this origin dates 
back to the remotest geological times. 
The first fresh water deposits known are those of the 
Devonian period, and the Old Red Sandstone of Kiltorcan 
contains a fossil bivalve allied to Anodonta. But though the 
fossils found may be closely allied to forms still living, and so 
may point to a far earlier origin than the Devonian period, yet 
it is possible that the Devonian deposits do actually contain 
the remains of the first fresh water fauna. The fact that 
species of Estheria, which is now an exclusively fresh water 
genus, as are all the Phyllopods, are found in Devonian deposits 
containing also brackish water species seems to indicate the 
beginning of colonisation. Species of Estheria are also found 
in brackish deposits in the Keuper of Schambelen, so that even 
our most characteristic fresh water forms may not have left the 
sea till Mesozoic times. 
It has been suggested that the fresh water fauna of the 
present day may be separated into old and new, the old fauna 
comprising all those widely distributed groups whose sojourn 
in fresh water dates back to remote geological times, while to that 
has been added a new fauna of immigrants from the sea in more 
recent periods. It is convenient and possible to distinguish 
certain genera and species which evidence shows to be of 
recent introduction, but such a distinction cannot be pressed 
very far, and it is necessary to regard the fresh water fauna as 
an assemblage of animals which has constantly been added to 
by recruits from the sea, and is still receiving them. 
It is possible that the immigration was much more extensive 
