6 BULLETIN 201, UNITED STATES NATIONAL MUSEUM 



northern coasts of Norway, Sweden, and Russia, where Arctic condi- 

 tions prevail. My sis mixta represents the Arctic-Boreal shallow- 

 water fauna. It penetrates southward on the Atlantic coasts of 

 America as far as the shores of New England, and on the European 

 side to the Skagerrack and the Baltic. The American and European 

 areas of distribution are connected by way of Greenland, Iceland, and 

 other Boreal areas where a shallow-water fauna is to be found. 

 Heteromysis formosa is known from Great Britain and the coast of 

 Norway. 



Out of a total of 22 genera represented in the combined areas, 7 

 are common to both sides of the Atlantic, out of 12 known from the 

 east coast of American and 14 from the Atlantic coast of Europe. 

 The American fauna lacks entirely such characteristic European 

 genera as Hemimysis, Paramysis, Praunus, Mesopodopsis, and Lep- 

 tomysis. 



Deep-water bottom-living species of the continental shelf. — Turning 

 now to the deeper water of the continental shelf, from roughly 100 to 

 1,000 fathoms, we find that 22 bottom-living species have been recorded 

 from the East American area. Of these, no fewer than 15 are known 

 also from the continental slope of Europe, off Norway and the west 

 of Ireland. These 15 species are : 



Hansenomysis fyllae Amblyops abbreviata 



Boreomysis tridens Dactylerythrops bidigitata 



Boreomysis arctica Meterythrops robusta 



Paramblyops rostrata Erythrops erythrophthalma 



Michthyops parva Erythrops microps 



Pseudomma affine Erythrops abyssorum 



Pseudomma rosetim Stilomysis grundis 

 Pseudomma truncatum 



Of these 15 species, 5 are known also from the Pacific continental 

 slope off the west coast of North America, namely : 



Boreomysis arctica Meterythrops robusta 



Pseudomma truncatum Stilomysis grandis 



Amblyops abbreviata 



In his account of the Ingolf Mysidacea Hansen (1908a) carefully 

 distinguishes between the species of the warm and cold areas of the 

 Arctic Ocean, the isotherm of 0° C. as a bottom temperature serving to 

 separate the two areas. From the warm area he notes 17 species of 

 bottom-living mysids, and of these 17 no fewer than 13 are included 

 in the above list, and all of them are known from the continental slope 

 of western Europe. It would appear, therefore, that there is a warm- 

 area fauna in the deep water of the Arctic which is more or less cir- 

 cumpolar in distribution and which penetrates deeply into the Atlan- 

 tic on both sides, and to a lesser extent into the Pacific. The striking 

 similarity between the fauna of the continental slope on both sides of 



