FRESH-WATER CRUSTACEA OF THE UNITED STATES. 643 



111 this couatry, this species was first found la the stomachs of the 

 whiteilsh. Subsequently, it was dredged in Lake Michigan by Drs. Hoy 

 and Stimpsou, in 40 to 50 fathoms, olf Racine. In Lake Superior, I 

 found it in a large number of the dredgiugs. It was. brought up with 

 sand and mud from 12 to 14 fathoms among the Slate Islands; from 4 to 

 6 fathoms in the cove at the eastern end of Saint Igiiace; from 8 and 13 

 fathoms, with Cladophora, &c., on the south side of the same island ; and 

 in many of the hauls from 72 to 148 fathoms. Mr. Milner also dredged 

 it, in 1872, in 60 fathoms, off Outer Island. It apparently furnishes a 

 large part of the food of the white-fish in many parts of the lakes. Nine- 

 tenths of the contents of the stomachs of white-fish taken at Outer Island 

 were made up of Mysis. 



I have carefully compared American with European specimens, and 

 with the beautiful figui^es given by Dr. G. O. Sars, in his elaborate 

 work on the Fresh- Water Crustacea of Norway, and am still unable 

 to detect any characters by which to distinguish them. The form and 

 ornamentation of the appendages seem to be exactly the same through- 

 out, and the habits appear to be the same in the Scandinavian lakes and 

 in Lake Superior. This species is also very closely allied to 3Iysis oculata 

 Kroyer, a marine species found on the coasts of Labrador and Green- 

 land. Loven points out this close affinity, and regards it, together with 

 the occurrence with it in the Scandinavian lakes of Gam mar acanthus 

 loricatus Bate, Pontiporeia affinis Lindstrom, and Idotea enfomon Fa- 

 bricius, all of which he regarded as specifically identical with previously- 

 known marine forms, as ev^idence that the lakes where it is found were 

 formerly filled with salt-water; that they had been cut oif from the sea 

 by the elevation of the Scandinavian peninsula ; and that the differences 

 between these species of the lakes and their allies of the neighbor- 

 ing ocean have been brought about by gradual changes in the habitats 

 of the lake-species. Dr. Sars adopts Loven's view as to the origin of 

 these species in the Scandinavian lakes ; regards the fresh- water Jllysis 

 as only a variety of the marine form ; and considers, with good reason, 

 the Gamynaracanthus as a distinct variety of the marine species. He 

 also points out the interesting fact that the slight differences (princi- 

 I)ally in the form of the telson) which distinguish the fresh-water from 

 the marine form of the Mysis are exactly such as distinguish immature 

 from adult individuals of the marine form, and are such differences as 

 might have been brought about by a slight retardation of development, 

 caused by the gradual change from a marine to a less congenial fresh- 

 water habitat. Dr. Sars gives the Gulf of Bothnia as a habitat of the 

 fresh-water variety, so that it is apparently not wholly confined to the 

 fresh waters, but, like the Pontoporeia affinis, lives also in the some- 

 what brackish waters of the Baltic. 



The occurrence, in Lake Michigan and Lake Superior, so far removed 

 from the sea, of this Mysis, and other forms so nearly identical with 

 marine species, is a fact of peculiar interest, which goes far toward 



