Reptiles, the class Reptilia, are an evolutionary grade of animals, comprising today's turtles, crocodilians, snakes, lizards and tuatara, as well as many extinct groups. A reptile is any amniote (a tetrapod whose egg has an additional membrane, originally to allow them to lay eggs on land) that is neither a mammal nor a bird. Unlike mammals, birds, and certain extinct reptiles, living reptiles have scales or scutes (rather than fur or feathers) and are cold-blooded. Advocates of phylogenetic nomenclature regard the traditional category 'Reptilia' to be invalid, and prefer to use the 'Amniota' or rather 'Sauropsida' category, because not all descendants of a common ancestor are included. However, in practice, these non-cladistic classifications, such as reptile, fish, and amphibian, remain in use by some biologists, especially in popular books written for a general audience. The historically combined study of reptiles and amphibians is called herpetology.