Barriers to Dispersion of River Fishes 317 



due to glacial influences is the presence in the deep waters of the 

 Great Lakes of certain marine types,* as shown by the explora- 

 tions of Professor Sidney I. Smith and others. One of these is a 

 genus of fishes, t of which the nearest allies now inhabit the Arctic 

 Seas. In his review of the fish fauna of Finland, J Professor A. J. 

 Malmgren finds a number of Arctic species in the waters of Fin- 

 land wliich are not found either in the North Sea or in the southern 

 portions of the Baltic. These fishes are said to " agree with their 

 'forefathers' in the Glacial Ocean in every point, but remain 

 comparatively smaller, leaner, almost starA^ed." Professor Loven§ 

 also has shown that numerous small animals of marine origin are 

 found in the deep lakes of Sweden and Finland as well as in the 

 Gulf of Bothnia. These anomalies of distribution are explained 

 by Loven and Malmgren on the supposition of the former con- 

 tinuity of the Baltic through the Gulf of Bothnia with the Glacial 

 Ocean. During the second half of the Glacial Period, according 

 to Loven, "the greater part of Finland and of the middle of 

 Sweden was submerged, and the Baltic was a great gulf of the 

 Glacial Ocean, and not connected with the German Ocean. By 

 the gradual elevation of the Scandinavian Continent, the Baltic 

 became disconnected from the Glacial Ocean and the Great 

 Lakes separated from the Baltic. In consequence of the gradual 

 change of the salt water into fresh, the marine fauna became 

 gradually extinct, with the exception of the glacial forms men- 

 tioned above." 



It is possible that the presence of marine types in our Great 

 Lakes is to be regarded as due to some depression of the land 

 which would connect their waters with those of the Gulf of St. 

 Lawrence. On this point, however, our data are still incomplete. 



To certain species of upland or mountain fishes the depression 

 of the Mississippi basin itself forms a barrier which cannot be 

 passed. The black-spotted trout, || very closely related species 



* Species of My;is and other genera of Crustaceans, similar to species 

 described by Sars and others, in lakes of Sweden and Finland. 



t Triglopsis thompsoni Girard, a near ally of the marine species Oncocottiis 

 quadncornis L. 



X Kritisk Ofversigt af Finlands Fisk-Fauna, Helsingfors, 1863. 



§ See Giinther, Zoological Record for 1864, p. 137. 



II Salnw fario L., in Europe; Salmo labrax Pallas, etc., in Asia; Salmc 

 gairdneri Richardson, in streams of the Pacific Coast; Salmo perryi. in J^pan; 



