US toy maker Hasbro has filed a federal trademark lawsuit against US cereal giant Kellogg, claiming a promotion on boxes of Kellogg's Frosted Flakes too closely resembles Hasbro's Memory card game. The back of the cereal box shows 16 cards featuring eight characters from the forthcoming film "Finding Nemo". Children are instructed to cut the cards out, turn them face down and then flip them back over two at a time to find matching pairs.