Cypripedium passerinum, a trans-continental boreal species, is known from the north Shore of Lake Superior, near the Pic River mouth, and might conceivably be found in northernmost Michigan. It is quite hairy, looking rather like a miniature Cypripedium reginae, but with a much smaller flower lacking any pink coloration.

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