126 Barriers to Dispersion of River Fishes 
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,} Professor A. J. 
Malmgren finds a number of Arctic species in the waters of Fin- 
land which 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 starved.” Professor Lovén§ 
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 Lovén 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 Lovén, ‘‘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 Mysis and other genera of Crustaceans, similar to species 
described by Sars and others, in lakes of Sweden and Finland. 
+ Triglopsis thompson: Girard, a near ally of the marine species Oncocottus 
guadricornis L. 
{ Kritisk Ofversigt af Finlands Fisk-Fauna, Helsingfors, 1863. 
§ See Giinther, Zoological Record for 1864, p. 137. 
{| Salmo fario L., in Europe; Salmo labrax Pallas, etc., in Asia; Salmo 
gairdnert Richardson, in streams of the Pacific Coast; Salmo perryi, in Japan; 
