president’s address. 
477 
Ireland. Lough Neagh, Lough Erne, and Lough Corrib. 
Finland. Lakes Ladoga and Onega, and various lakes in 
the neighbourhood, as well as in parts of the Baltic. 
Sweden. Lakes Wener, Wetter, Malar, etc. 
Norway. Lake Mjosen. 
Denmark. Fureso. 
Germany. Mauer, Dratzig, Madu, Tollensee. 
North America. Lakes Michigan, Superior, Erie, and 
Green Lake. 
It is clear that M. relicta has been evolved from M. oculata, 
and there is evidence that the change occurred in Postglacial 
times. As regards the Baltic region, no doubt M. oculata 
inhabited the Yoldia sea together with other arctic forms, and 
was transformed into M. relicta during the period which ended 
with the changing of the Yoldia sea into the fresh water 
Ancylus Lake. The hydrographic conditions permitted M. 
relicta to penetrate into the water system of Germany which 
now drains into the Baltic, but not into those lakes which 
now communicate with the North Sea , 1 and it became established 
in the Scandinavian and Finnish lakes, which have since been 
shut off by earth movements from the Baltic. The transforma- 
tion of the two species has evidently occurred independently 
in Ireland and North America at about the same time, the 
change being connected with the melting of the glaciers and 
the earth movements, and decreased salinity associated there- 
with. 
One further group of marine animals in fresh water should 
be mentioned. This comprises a number of species of Isopod 
Crustacea found in springs and caves, and adapted to a 
subterranean life. The genus Typhlocirolana, for example, is 
scarcely distinguishable from the marine genus Cirolana except 
by the absence of eyes, and it consists of a few species found 
in springs in Algeria and the Balearic Islands. The parent 
genus Cirolana is not only marine, but is usually found in 
comparatively deep water. Such animals as these, and there 
1. Samter. Auhaug za den Abh. du K. Preuss. -Vkad. Wiss, 1905. 
VOL. IX. 
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