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d. Their skins are tough, and are studded in most of them with denticles (placoid scales), which are not homologous with the scales of bony fishes, for both dermis and epidermis take part in their formation, instead of the dermis alone. The teeth of the sharks and rays represent placoid scales that are modified and are embedded in the gums alone, not in the jaws.

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Mammals are any vertebrates within the class Mammalia (/məˈmeɪli.ə/ from Latin mamma "breast"), a clade of endothermic amniotes distinguished from reptiles and birds by the possession of a neocortex (a region of the brain), hair, three middle ear bones and mammary glands.

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Reptiles are cold-blooded vertebrates. (Vertebrates have backbones.) They have dry skin covered with scales or bony plates and usually lay soft-shelled eggs.

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Birds (Aves), also known as avian dinosaurs, are a group of endothermic vertebrates, characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a lightweight but strong skeleton