Similar species: Small Yellow Lady's Slipper (Cypripedium parviflorum var makasin) - Smaller; purplish petals behind the "slipper". Small White Lady's Slipper (Cypripedium candidum) - Small, white flowers; greenish petals. Hybrid Lady's Slipper (Cypripedium parviflorum X C. candidum) - Small, white flowers; purplish petals.
Large Yellow Lady's Slipper. The petals are green and the flower is bigger. This variety grows in woods.
A group of Large Yellow Lady's Slipper in the woods in mid-June.
Flower just starting to open.
Seed capsule from previous year. This photo was taken in January. (I don't know which variety of Yellow Lady's Slipper this is, but they both look the same at this time of year!).