How is a platypus considered a mammal if it lays eggs?

Because they have hair and the mammalian middle ear, which consists of two of the jaw bones found in reptiles and birds. These two jaw bones are the quadrate and the articular. Platypuses have retained the ancestral condition of egg laying, but they are unmistakably mammalian because of their unique mammalian middle ear. No reptile has hair or the mammalian middle ear.