500 FAUNA CANADENSIS. 



ceans belonging to other marine genera, sucli as Mysis, IdoteCt, and 

 Gammar acanthus ; and in Lakes Superior and Michigan it is associated 

 lilcewise with Mysis. Both in Scandinavia and in the Ujoper Lakea 

 of North America the species of Mysis is the same, viz., M. relicta, 

 Loven — a species which is regarded by its describer as being identical 

 with the Mysis oculata, Fabricius, of both sides of the Atlantic. 

 Upon these facts, Loven founded the theory that the Scandinavian 

 lakes had formerly been part of the sea, frotn which they had been 

 cut off by geological changes resulting in the elevation of the 

 Scandinavian peninsula. He believed that the Mysis and other 

 species of Crustaceans belonging to marine genera had originally 

 inhabited salt water, but that they had been able to survive and 

 accommodate themselves to the change by which the Swedish lakes 

 were gradually and slowly converted into fresh water, whilst other 

 less plastic forms of marine life had died out. Lastly, he believed that 

 the differences between the Crustaceans of the lakes and the allied 

 species of the neighbouring ocean were due to the modifications 

 which the former had undergone in adapting themselves to live in 

 fresh water. 



Without entering into the merits of Loven's theory, it is extremely 

 interesting to find that Lakes Superior and Michigan have yielded at 

 any rate two of the Crustaceans of the Scandinavian Lakes, namely, 

 Pontoporeia ajfinis and Mysis relicta. The last mentioned species, 

 curiously enough, does not appear to exist in Lake Ontario, though 

 plentiful in Lake Superior ; probably because the conditions present 

 in the latter lake are somehow more suitable for it. The other 

 species, however, viz., Pontoporeia affinis, if rightly determined, 

 abounds in the deeper parts of Lake Ontario. 



As before remarked, the Crustacea must be left for the present 

 undescribed ; but the following list indicates the forms which were 

 discovered in Lake Ontario, and the habitat of each. 

 8. Gammarus, sp. 



A small fresh- water shrimp, varying in length from one-fifth to one 

 quarter of an inch, and of a greenish-brown colour during life, with a 

 dark green intestinal tract. The antennae and antennules are about 

 half the length of the body, and nearly equal. 



Hah. — Numerous examples of this pretty little species occurred 

 amongst CharcB and other aquatic plants in depths of from one to 

 three fathoms. 



