644 REPOET OF COMMISSIONER OF FISH AND FISHERIES. 



proviug the marine origin of a part of the fauna of our great lakes. Dr. 

 Stiinpsou,* in his first notice of his dredging in Lake Michigan, while 

 regarding the Mysis as a new species, recognized its close affinity with 

 " certain arctic forms," and supposes the same changes to have taken 

 place in Lake Michigan as in the Scandinavian lakes. He says, "il/j/si* 

 is a marine genus, many species of which occur in the colder parts of the 

 ^orth-Atlautic seas. One species, M. relicta, was found by Loveu in 

 company with Idothea entomon and other marine Crustacea in the deep 

 fresh-water lakes Wener and Wetter of Sweden, indicating that these 

 basins were formerly tilled with salt-water, and have been isolated from 

 the sea by the elevatory movement of the Scandinavian peninsula, 

 which is still going on. That the same thing has occurred in our own 

 lakes is shown by the occurrence in their depths of the genus Mysis, 

 notwithstanding the non-occurrence of marine shells in the Quaternary 

 deposits on their shores. Kingston, on Lake Ontario, is, I believe, the 

 highest point in the valley in which such shells have been found. Very 

 probably, at the time when the sea had access to these basins, the com- 

 munication was somewhat narrow and deep, and the influx of fresh 

 water from the surrounding country was sutficient to occupy entirely the 

 upper stratum, while the heavier sea-water remained at the bottom. 

 After the basin had become separated from the ocean by the rise of the 

 land, the bottom water must have become fresh by diffusion very slowly 

 to allow of the gradual adaptation of the crustaceans to the change of 

 element." In the entire absence of geological evidence of any oceanic 

 connection with Lake Superior in recent geological times, the occur- 

 rence, in its otherwise strictly lacustrine fauna, of a very few forms of 

 life showing close affinity with marine species, seems scarcely to war- 

 rant so positive an assumption of such a connection. At the time Lake 

 Ontario was a part of the great Saint Lawrence Valley sea, there was, 

 very likely, no insuperable barrier in the Niagara River to the npward 

 migration of active swimming animals like Mysis, and some of the 

 inhabitants of the upper lakes may have reached theirpreseut homes 

 by this route, during the northward movement of the fauna, at the 

 close of the Quaternary epoch. On the other hand, 3Iysis relicta, 

 although originally derived from the strictly marine species 21. oculata, 

 may have existed long enough to have had the same history as some of 

 the strictly fresh- water species, known to be common to Northern Amer- 

 ica and Northern Europe, since it has much the same geographical dis- 

 tribution. The investigation of the fauna of the lower lakes and Lake 

 Champlain, and possibly of Lake Winnipeg, will throw much liglit upon 

 these interesting questions, and it seems best to reserve any lengthy dis- 

 cussion of them until such investigations have been made. 



Whether we should regard the fresh-water form of Mysis as a variety 

 of M. oculata or as a distinct species, seems a matter of little import- 



*Oii the Deep-Water Fiiuua of Lake Michigan, American Naturalist, vol. iv, p. 403, 

 September, 1870. 



