The classification for mammals that lay eggs is monotremes. This unique group includes species such as the platypus and echidnas. Unlike most mammals, which give birth to live young, monotremes reproduce by laying eggs. They possess characteristics of both mammals and reptiles, including fur and the ability to produce milk. Monotremes are significant for understanding mammalian evolution, as they represent a branch of the evolutionary tree that showcases distinct reproductive adaptations. Their study sheds light on the diversity of reproductive strategies within the animal kingdom.