Toothwort

Yeah, the Audubon guide is good to use like a mugshot book, but not good for IDing things by features other than color and general flower shape. Have you tried Newcomb's Wildflower Guide? It allows you to key out flowers based on a number of features. It looks really intimidating at first, but you'll be surprised how quickly you get the hang of it. The only major problem is that many of the specific names are out of date. For example, this plant is called Dentaria diphylla. Still, well worth the money.

It's a toothwort. Probably Cardamine diphylla: http://www.ct-botanical-society.org/galleries/cardaminediph.html