The subclass of mammals characterized by the ability to lay eggs is called monotremes. This unique group includes the platypus and several species of echidna. Monotremes are distinct from other mammals, which mostly give birth to live young. They exhibit reproductive features of both reptiles and mammals. For example, monotremes lay eggs that have leathery shells, similar to reptiles. The study of monotremes provides insights into the evolutionary history of mammals and the adaptations that enable them to thrive in their respective environments.