I think that this is Virgin's Bower and not Autumn Clematis. The Connecticut Botanical Society's web site http://ct-botanical-society.org/galleries/clematisvirg.html says that the main difference is the leaves.

"Virgin's bower is easily confused with sweet autumn clematis (Clematis terniflora or Clematis paniculata), an Asian vine that has escaped from gardens. The two vines can be distinguished by their leaves; on a virgin's bower, almost all leaves are have jagged teeth. Sweet autumn clematis has rounded leaves, which are mostly untoothed."

Though its not a great shot, I think that these leaves are toothed. Thanks to Constance for getting me to the right family.