Trochonema Vs Holopea Gastropods

[minnbuckeye]

A nature center in Rochester Mn has a paleontology program for local schools that is very popular. During the last year, I have purposely collected fossils totally WITHIN the city limits of Rochester and am in my final stages of preparing an exhibit to present to the center as a way to show students what is available to them in their own back yards. I had a label made identifying some gastropods as Trochonema. But after stumbling onto a web site, I now am questioning whether they are Holopea. Can anyone explain to me how to differentiate these gastropods from each other? Thanks for any input.