The class that contains the first entirely terrestrial vertebrates, which can live in dry terrestrial habitats as their skin is covered by a layer of horny scales, preventing water loss. They breathe atmospheric oxygen by means of lungs assisted by respiratory movements principally involving the ribs (there is no diaphragm). Reptiles are cold-blooded (see poikilothermy) but behavioural patterns make it possible for them to maintain a fairly even body temperature throughout the day. Fertilization is internal and the majority of reptiles lay eggs on land. These eggs have a porous shell to provide protection from desiccation and allow gas exchange. In some reptiles the eggs are retained within the body of the mother until the young are ready to hatch, thereby greatly reducing juvenile mortality (see ovoviviparity).

The class includes the modern crocodiles, lizards and snakes (see squamata), and tortoises and turtles, as well as many extinct forms, such as the dinosaurs and Pterosauria. This grouping is paraphyletic because it excludes the birds, descendants of theropod dinosaurs. Hence, contemporary classification schemes generally include birds as a constituent group of the reptiles.

http://www.tolweb.org/Amniota/14990 This Amniota page from the Tree of Life project shows the phylogenetic relationships of reptile groups, birds, and mammals