40 The MoUuscan Family Planorbidae 



Galente, Antigua, St. Christopher, St. Thomas, Puerto Rico, Haiti, Ja- 

 maica, and Cuba. The three genera Australorbis, Tropicorbis, and Dre- 

 panotrema inhabit most of the islands mentioned. Another route is from 

 Venezuela northward up the eastern coast of Panama, close to Costa Rica, 

 Nicaragua, Honduras, Guatemala, eastern Mexico to the gulf coast of the 

 United States. The genus Tropicorbis has this distribution and many of its 

 species are widely distributed. Some migrant birds pass up the west coast 

 of Central America and ]\Iexico. 



It is the writer's belief that the populating of many if not all of the 

 islands of the West Indies with planorbid and other fresh-water mollusks 

 has been brought about through the agency of migrating birds. They stop 

 at these islands to feed and rest, the snails become attached to the feet or 

 other parts of the bodies of the birds, to be liberated when the birds make 

 other stops. Young snails have been found attached to the feet of many 

 birds. In the case of the Basommatophorous mollusks a single snail in- 

 troduced into a pond would be sufficient to insure population since self- 

 fertilization is easily accomplished. By this means in a few years a body 

 of water would be a prolific habitat for the species. Aquatic insects, 

 especially beetles, would perform the same service, especially in populating 

 closely associated ponds or streanis. Young snails have been found attached 

 to the wings (elytra) of beetles. 



Natural migration doubtless takes place in connected streams and other 

 bodies of water and will account for the dispersal of related species within 

 the drainage of a water system. Floods often carry species of snails over 

 natural divides. In the distribution and dispersal of the Mollusca, all 

 agencies must be taken into account. The scarcity of records of snails on 

 the bodies of migrating birds is primarily due to the fact that those who kill 

 the birds, sportsmen and hunters, pay no attention to what may be at- 

 tached to the feet of their victims. Ornithologists, too, do not carefully ex- 

 amine the bodies of the specimens and the snails fall off or are brushed off 

 during the preparation of the skin for the cabinet. Careful examination of 

 the bodies of birds would doubtless provide many valuable evidences for 

 avian dispersal of fresh-water and other mollusks. 



