Yet another megadeal fell through as American Home Products Corp.'s $35 billion merger with Monsanto Co. was called off last week. In January, AHP's bid to merge with SmithKline Beecham also failed. The Monsanto deal, rated the biggest merger yet in the pharmaceutical industry, would have created the nation's largest marketer of prescription drugs. Analysts predicted AHP and Monsanto would seek new merger partners. AHP stock closed at $48.94 Friday, down 0.6% from the week before.