FRESH- WATER SCREW. 153 



for the fresh- water habit, and a little difference in the 

 shade of colour, the latter being of a yellowish, instead 

 of a bluish grey, is so absolutely identical with the 

 Locust Screw, that, notwithstanding it is registered 

 under a name of its own, Gammarus pulex, it is diffi- 

 cult to believe that the two are not of common parent- 

 age. Certain facts observed in Sweden give colour to 

 such a suspicion. The large inland seas of fresh water, 

 Lakes Wetter and Wener, in the south of that country, 

 are situated on high ground, and have the surface of 

 their water 300 feet above the level of the Baltic, 

 whereas the bottom is 120 feet below such level In 

 these lakes (which appear to have been lifted up with 

 the gradual uprising of the country) have been found 

 several genera and species of Crustacea, three of which 

 are AMPHIPODA, which are affirmed to be identical with 

 marine ones, viz., GammaracantJms loricatus (Sabine, 

 Eoss, Kroyer), Pontoporeia affinis (Lindstrom), and 

 Gammarus cancelloides (Gerstfeldt). 



The first is now only known to exist in the Arctic 

 seas, the second in the Baltic, and the last was found in 

 Lake Baikal, in Central Asia. It is therefore suggested 

 by Loven, that the land was raised so as to convert 

 these waters from marine bays into inland lakes. These 

 marine species were retained within the basins, the 

 waters of which have since been changed, through the 

 agency of springs, into fresh water ; and with the gra- 



