The Class Reptilia is a grade comprising the scaly tetrapod descendants of Carboniferous Cotylosaurs. It include all of the extant "scaly" orders, Chelonia (turtles), lepidosaurian Rhyncocephalia (Sphenodon) and Squamata (lizards & snakes), and archosaurian Crocodilia. Note that the archosaurian Dinosaurs properly includes only two terrestrial orders, the Ornithischia and Saurischia, and that other groups of large, extinct reptiles, including aquatic (Plesiosaurs & Icthyosaurs), aerial (Pterosaurs), and terrestrial (Pelycosaurs) forms, are not "dinosaurs". The Class Reptilia is a grade because of the exclusion as separate taxonomic classes of "feathery" birds (Aves), which are descended from dinosaurs, and "hairy" Mammalia , which are descended from the therapsid lineage. [Note that mammals and birds are probably more closely related to each other than either is to turtles].