Cypripedium

1. Leaves all basal or nearly so; lip split down the middle on the upper side.

C. acaule

1. Leaves cauline; lip with a small opening only at the base.

2. Lip pale to deep yellow.

C. parviflorum

2. Lip variously white and purple.

3. The 3 sepals all distinct nearly to base; lip prolonged below to a blunt conical pouch, strongly marked throughout with purplish.

C. arietinum

3. The 2 lateral (or lower) sepals fused most or all of their length; lip smoothly rounded without conical prolongation below, entirely white outside or with extensive white areas.

4. Lip entirely white outside, up to 2.5 cm long; sepals and lateral petals greenish yellow; plants with 3–4 leaves crowded near middle of stem, sparingly puberulent.

C. candidum

4. Lip white, marked with pink or purple outside, 2.5–5 cm long; lateral petals and sepals white; plants with more leaves, the stem and leaves copiously hirsute.

C. reginae

All species found in Cypripedium

Cypripedium acauleMOCCASIN FLOWER, PINK LADY-SLIPPER, STEMLESS LADY-SLIPPER 
Cypripedium arietinumRAM'S HEAD LADY-SLIPPER 
Cypripedium candidumWHITE LADY-SLIPPER 
Cypripedium parviflorumYELLOW LADY-SLIPPER 
Cypripedium reginaeSHOWY LADY-SLIPPER, QUEEN'S LADY-SLIPPER 

Citation:

MICHIGAN FLORA ONLINE. A. A. Reznicek, E. G. Voss, & B. S. Walters. February 2011. University of Michigan. Web. May 26, 2017. http://michiganflora.net/genus.aspx?id=Cypripedium.