Dryad's Saddle, or Polyporus squamosus, is an edible basidiomycete mushroom found in the genus Polyporus. Overeducated people who have taken too many classical studies credits have given the name "Dryad's Saddle" to this mushroom because of creatures in Greek Mythology called Dryads who could apparently ride on the kidney-shaped top. Alternatively, confused animal hunters have called it "pheasant's back mushroom" due to the similarity to a pheasant from a distance.
This guy is common and widespread, being found west of the Rocky Mountains in the United States and over much of Western Europe. It commonly fruits in the spring, occasionally during autumn, and rarely during other seasons. I stumbled over this while looking for Morels during the spring. Both have similar fruiting times and Dryad's Saddle is very conspicuous with sizes of up to 2 feet across. It plays an important role in woodland ecosystems by decomposing wood, usually elm, but is occasionally a parasite on living trees. It produces "white rot", which means that it can digest lignin in addition to cellulose found in wood.
This mushroom is commonly attached to dead logs or stumps at one point with a thick stem. Generally, the fruiting body is kidney shaped to circular with dimentions of 3-12 inches across and up to 4 inches thick. The body can be yellow to brown and has "squamules" or scales on its upper side. On the underside one can see the pores that are characteristic of the genus Polyporus, which will produce a white spore print if laid onto a sheet of paper. They can be found alone, in clusters of two or three, or forming shelves. Young specimens are soft but toughen with age. You can check out some pictures (~500kb each) that I took here:
While Dryad's Saddle is certainly not poisonous, it is generally not prized as an edible. Cookbooks dealing with preparation generally recommend gathering these while young, slicing them into small pieces, and cooking over low heat. Most people find the "mealy" favor displeasing, although some will eat it with impunity. More people value the thick, stiff paper that can be made from this and many other mushrooms of the genus Polyporus.
This article stemmed from my original posting at Wikipedia. It's my own work, and it's still under the GFDL.