So... any particular reason this wouldn't be Clematis terniflora? From what I can see here, it fits that species perfectly, but is not any of the similar species (Clematis virginiana, Clematis ligusticifolia, Clematis drummondii...) I'm aware of.
Is your Clematis more of a shrub type, or vine? how large you say it is? And can you get a photo of the leaves? The reason for all the questions is it could be one the C. Heracleifolia group. Here's a link to the C. Heracleifolia Mrs. Robert Brydon which looks close to yours. http://www.clematis.com.pl/wms/wmsg.php/1061.html
That is a great website, Gldandrews! I wish their photos were bigger though. My eyes don't seem to focus on small pics anymore. I just can't seem to see the detail. But after a google search, you sure nailed it. exactly! Thanks so much.