Saute the sliced mushrooms in the butter in a skillet until delicately browned.
Add the flour and salt, and blend.
Add the milk and cook, stirring constantly, over low heat until smooth and thickened.
Meanwhile heat the oysters in a saucepan over low heat until edges just begin to curl.
Add to creamed mushrooms with the pepper and celery salt, and mix thoroughly.
Serve garnished with toast points, made by cutting each toast slice in halves from corner to corner.