[Illustration: Fig. 14--The Four-horned Sting-fish (_Cottus quadricornis_), reduced from Professor Smitt's figure in the _Fishes of Scandinavia_.]
This disposes, therefore, of Professor Credner's main criticisms. As for the fauna of the relict lakes, we are now only concerned with those of Northern Russia, Finland, and Sweden. In the lakes Wetter and Wener in the latter country occurs the four-horned sting-fish (_Cottus quadricornis_, Fig. 14), which, as we have learned, also inhabits the northern part of the Baltic, and, as was suggested, migrated there at a time when the latter was connected with the White Sea. The principal food of this little fish consists in a marine Crustacean called _Idotea entomon_, an animal allied to our common woodlouse. This is a typical marine species, but it occurs also in the relict lakes of the countries mentioned above, as well as in the Baltic and the Caspian. Perhaps the best known form with a similar range is the Schizopod crustacean _Mysis relicta_[3] (Fig. 15), which is clearly a descendant of the Arctic marine _Mysis oculata_, of which it was formerly considered a mere variety. The two Amphipods _Gammaracanthus relictus_ and _Pontoporeia affinis_ and the Copepod _Limnocalanus macrurus_, are three additional well-known Arctic crustaceans whose range differs but little from those above-mentioned.[4]
[Illustration: Fig. 15--Mysis relicta, a small shrimp-like Crustacean, after Sars (enlarged).]