I'm probably not an expert, but I do a pretty good job researching things:
That brings us to the Cypripedium calceolus complex, the yellow lady's-slipper of Europe, and, atone time not too long ago, of North America, until J. T. Atwood, in 1985, pronounced the American yellow-lipped slippers distinct from the European yellow calceolus, a belief quickly sanctioned by most orchid experts in North America, as well as Phillip Cribb, the English authority at Kew. This separation eventually resulted in Sheviak's formation of the North American taxa Cypripedium parviflorum var.parvif lorum, C. parviflorum var.pubescens, and C. parviflorum var.makasin, (NANOJ 2(4):319): one species with three varieties.
For a detailed explanation of the taxonomic confusion - and resolution - in the yellow slipper complex, one should consult two excellent authors: the
aforementioned The Genus Cypripedium by Phillip Cribb. And Charles Sheviak who has written several articles in the American Orchid Society Bulletin in 1994 and 1995\(NANOJ 2(4):319). The degree of resolution, however,as they freely admit, is subject to change with further notice
From a post by Charles Sheviak:
Cypripedium parviflorum is the most widely-ranged North American species. It is transcontinental in the North, ranging south through most the southeastern U.S and in the mountains of the West nearly to Mexico. It is also the most variable and taxonomically complex of our species. At present three varieties are recognizable, but this taxonomy does not accommodate all variation and some plants cannot be assigned to an infraspecific taxon.
The most widespread variety is var. pubescens, which occurs virtually throughout the range of the species. This is also the most variable variety. In the eastern deciduous forests the plants are large with large flowers, and similar plants occur in the north and west. Commonly the sepals and petals are rather pale with highly variable darker markings. From the Great Lakes northward and westward, however, the plant is much more variable, with a tremendous range of size and shape of flowers and floral parts. Small northern plants were described as variety planipetalum, because their small flowers commonly have mostly unspiralled petals. These plants are merely extreme forms of var. pubescens, and diverse populations of var. pubescens include a great range of forms. Many plants can be phenotypically manipulated by growing them under different conditions; cool conditions induce small growths bearing small flowers with flat petals. The point here is that many such plants will develop into larger var. pubescens in cultivation; var. planipetalum does not exist.
The other varieties of C. parviflorum have small flowers. In the forests of the eastern U.S. occurs var. parviflorum, often growing on rather high, drier slopes. The sepals and petals appear dark due to closely spaced, tiny spots. This plant is rather rare and not generally known in cultivation. The plant that is commonly known and cultivated as var. parviflorum is actually var. makasin. This is a more northern plant, occurring from the Great Lakes region northward into the taiga. It is typically a plant of calcareous fens, growing in open forests and openings. It also occurs in thickets in moist sands. The flowers bear uniformly dark sepals and petals, rather like C. calceolus. Typical plants of this variety are very rare west of central Canada; in the western mountains the variety seems to be represented by plants without the uniform dark sepals and petals.