Mix peanut butter and margarine; stir in cereal. Mix in powdered sugar, 1/3 at a time. Press mixture firmly in ungreased square pan, 9 x 9 x 2-inches. Heat shortening in 1 quart saucepan over low heat until melted. Add chocolate chips. Heat over very low heat, stirring constantly, until chocolate is melted and mixture is smooth. Spread chocolate mixture over cereal mixture. Refrigerate until chilled, about 1 hour. Remove from refrigerator 10 minutes before serving. Cut into bars, about 2 1/4 x 1 1/2-inches. Refrigerate only remaining bars. Yield: 24 bars.