listIndividuals versus listInstances

[Steve Vestal]

Given an OntModel myModel and an OntClass myClass contained in myModel, 
the call myModel.listIndividuals(myClass) provides the expected list of 
Individuals that are members of myClass; but the call 
myClass.listInstances() doesn't list anything.  I am curious what the 
difference is between the two (I was not able to make that out from the 
javadoc).