What are the key features of a mammals?

They have hair, a skeleton, 4 limbs, circulatory system w/4 chambered heart, and are endothermic. ---- There are five basic features which characterise mammals: * they are vertebrates * they have fur, skin or hair * they are warm-blooded * they breathe through lungs (not gills) * they bear live young (with the exception of the two monotreme types, which lay eggs) and the young feed on mothers' milk