White-leaf Leather-flower (Clematis glaucophylla) (subgenus Viornae) 2015-5-16 Mt. Cuba Center, New Castle County, Hockessin, DE

it only bears one solitary flower in between leaf sets on a 6" stem
when flowers open to a bell shape and matures it shows a pale yellow throat
http://www.mtcubacenter.org/plant-finder/details/clematis-glaucophylla/
many species in the subgenus Viornae are variable and hybridize easily
see:
http://clematisviorna.info/flowers.html
2015-5-16 Mt. Cuba Center, New Castle County, Hockessin, DE
Mt. Cuba Center also has Clematis viorna but that has different leaves and a different blooming habit
http://www.mtcubacenter.org/plant-finder/details/clematis-viorna/
another example of multiple flowers of C. viorna:
http://plants.usda.gov/core/profile?symbol=clvi4