Many of the big flowered ones are hybrids. In fact there are probably more hybrid Clems around than species.

Indeed Owdboggy

Most of the large flowered hybrids have Clematis patens in their ancestry. Don't know what the other species involved are though C. patens is a naturally large flowered species that was obviously deemed useful to early breeders.

Just searched and found this picture of the wild species Clematis patens in Japan.

http://www.clematisinternational.com/japan2008014.jpg