How is a platypus like a reptile?

The platypus is nothing like a bird.

It is an egg-laying mammal. The eggs it lays are soft-shelled and leathery, unlike a bird's eggs, which are hard-shelled. There are some general characteristics which are similar. Birds and platypuses are both air-breathing endothermic vertebrates. Birds and platypuses both have a cloaca, a single vent for both reproduction and waste.