4 BULLETIN 2 01, "UNITED STATES NATIONAL MUSEUM 



If we analyze the mysidacean fauna of America still further the 

 following facts emerge: 



Fresh-water species. — Four species of fresh-water mysids are known 

 from America. Of these Mysis relicta, found in the Great Lakes of 

 North America, is also found in the relict lakes of Europe, in Ireland, 

 England, Scandinavia, and northern Germany. It has probably been 

 derived from the very abundant and widely distributed Arctic ma- 

 rine species Mysis oculata, following isolation as the result of glacial 

 action. 



The two species of Antromysis, A. cenotensis and A. anophelinae, 

 found respectively in fresh-water caves in Yucatan and crab holes in 

 Costa Rica, are peculiar to these places and the genus is unknown 

 outside America. 



Diamysis americana, found in ditches in the Botanic Gardens at 

 Paramaribo, Dutch Guiana, is the American representative of a Medi- 

 terranean genus where its species occupy similar brackish and fresh- 

 water habitats. I have suggested that it is a survival of the old Tethys 

 Sea fauna, when the genus was probably widely distributed in that 

 area, and retired to the Mediterranean and to the fresh waters of 

 America when the Atlantic Ocean took on its present form. The 

 genus Antromysis is more nearly related to Diamysis than to any 

 other genus and may conceivably have been derived from members 

 of that genus that took to the more specialized habitats of under- 

 ground waters and crab holes. 



Littoral and coastal species. — On the Atlantic coast of America 

 from Labrador to the Straits of Magellan, 22 species of littoral my- 

 sids are known, and on the Pacific coast from Alaska to the Straits 

 of Magellan the number of recorded species is 30. Only one species 

 is common to the two coasts, Gastrosaccus mexicanus having been 

 taken on both sides of the Isthmus of Panama. The genera found 

 on both coasts are very similar and even identical, and the two 

 coastal faunas are very parallel when viewed from the standpoint 

 of the genera represented. 



If we compare the genera of east-coast American mysids with those 

 of the European coasts a certain similarity between the two faunas 

 can be discerned, but the parallel is not so close as that between the 

 east and west coasts of America. Table 1 sets forth not only the 

 genera with number of species represented off the east and west coasts 

 of America, but it furnishes a comparison between the east coast 

 of America and the Atlantic coast of Europe : 



