Distribution in Time and Space 39 



Ai(.str(dorbi.s is known from South America and the West Indies but 

 is not found north of the Isthmus of Panama. Tropicorbis occurs in South 

 and Central America, in Mexico, the West Indies, and in the southern 

 part of the United States bordering the Gulf of Mexico. Curiously enough, 

 a closely related group, known as Afroplanorbis, is found in northern and 

 central Africa and resembles Tropicorbis so closely, in both shell and 

 anatomy, that it is a question whether the two groups are not identical 

 generically. Anisus is mostly European and Asiatic. Segmentina is typi- 

 cally European, west Asian, and north African. Several groups related to 

 Segmentina, as Polypylis, Helicorbis, Pingiella, and Intha, are confined 

 to eastern Asia and some of the Pacific islands. Taphius occurs only in 

 Central America and in northern South America. Gyraulus is the most 

 widely distributed of any of the genera of Planorbidae, being known from 

 North America, Europe, Asia, Africa, and some of the islands of the 

 Pacific as w-ell as from the West Indies. It is typically a northern group 

 (palaearctic) but a few species extend southward into the tropical regions. 



The present study of the distribution of the members of the family 

 Planorbidae shows conclusively the value of the anatomy in the classi- 

 fication of the species. It is of interest to note that in considering the 

 distribution of the genera, details of the anatomy have been the only 

 means by which the different groupings could be made with certainty. 

 Hence our knowledge of distribution has become more definite and better 

 understood than was possible when shell characters were given chief taxo- 

 nomic value. The shell alone is not sufficient to distinguish groups because 

 there are parallels in growth tendency which render shells of widely sepa- 

 rated genera confusingly similar in appearance. Instances of this are found 

 in the species of Tropicorbis and Planorbida, Aiistralorbis and Helisonia, 

 Segmentina and Polypylis, etc. 



Dispersal of Species 



The presence of species of mollusks in isolated places has been somewhat 

 of a puzzle to many zoologists. How, for example, are ponds without inlet 

 or outlet and more or less widely separated, populated by one or more 

 species of planorbids or other fresh-water species? In what manner have 

 the same species of Tropicorbis, Australorbis, or Drepanotrema found 

 their way to so many of the islands of the West Indies, isolated by large 

 and deep areas of salt water? Examining the distribution of Aiistralorbis, 

 for example, we find it occurring in eastern and northern South America 

 and also in many of the groups of islands — the Lesser Antilles, the Greater 

 Antilles, Puerto Rico, and Jamaica. Some species of Tropicorbis have the 

 same distribution. 



Natural dispersal by migration is out of the question. Driftwood might 

 answer for land snails (at least to a limited degree) but not for fresh- water 

 forms. A former connection with South America occurred too remotely to 

 answer satisfactorily the question. When we examine the migration routes 

 of the migrating birds, especially the wading birds and ducks, we are 

 struck by the similarity of the geographic routes of the birds with the 

 geographic distribution of the snails. One route is from Venezuela, South 

 America, through the Caribbean and West Indian islands as follows: 

 Trinidad, Grenada, St. Vincent, St. Lucia, Martinique, Dominica, Marie 



