Thirteen sf stories -- at least five of them excellent, another three or four worthwhile. Among the best: Robert Thurston's grayly sinister variation on the amnesiac-locked-in-a-mysterious, room gambit; Gustav Hasford's ""Heartland,"" which turns a desperately bored housewife into a horse before the blase and uncomprehending eyes of her husband (""Horsefeathers. You'll be fine.""); a whimsical, irresistible story by R. A. Lafferty about an odd corner of existence where people appear to have mastered a benevolent form of Shylockian surgery.