Marcus,

I've never collected in South America except for Trinidad and
Tobago...There is (rarely in most spots) a largish freshwater neritid on
Trinidad that I called Neritina zebra.  This is probably your critter.  I
do not believe usnea occurs that far south.  I've seen population from
Central America that seem to intergrade with what Russell called piratica,
but .....anatomical studies are probably needed.....

Hope that helps....Kurtles

Mr. Kurt Auffenberg
Senior Biological Scientist
Exhibits & Public Programs
University of Florida
Hull Road & SW 34th Street
Powell Hall, P.O. Box 112710
Gainesville, Florida 32611-2710

Telephone: 352-846-2000, ext. 253
Fax:  352-846-0253