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If you mean the "Elm oyster", that would be Hypsizygus ulmarius, not a true oyster, but a great edible. It grows on a wide range of substrates, similarly to P. Ostreatus, and details can be found in Stamets Growing Gourmet and Medicinal Mushrooms. It's also one species that does well with peroxide methods used by Rush Wayne in his manual, Growing Mushrooms the Easy way, at http://members.aol.com/PeroxyMan/index.html.
This is a choice edible, and well worth the effort.