Wow, what a beautiful clematis. That is a keeper. Seedlings don't have cultivar names. You can call it whatever you want. If you would like to see photo's of similar clematis go to clematis on the web. They have a clematis picture search. Just select atragene group and white tepal color and it will show you lots of similar clematis.

Here is a link that might be useful: Clematis on the web

What a gorgeous clematis, is this its first year for flowering?

That picture was last years, the third year flowering, what is outside is now the fourth year. It sometimes gives me flowers, only one or two, late in the season as well. It grows under Etoile Voilette which turned out to be a great combination as it flowers early and Etoile Voilette is taller and flowers from mid summer on here. That trellis has color all season.
Dana

So it flowered it's first year?
That's excellent.

Lovely Dana. I agree that knowing its name is immaterial. Just call it your white macropetala from seed! If you absolutely have to know, search for white tepals and Atragene under the group on COTW and see what it best matches.

Here is a link that might be useful: COTW search