It was not until the beginning of the 19th century that it became clear that reptiles and amphibians are, in fact, quite different animals, and Pierre André Latreille erected the class Batracia (1825) for the latter, dividing the tetrapods into the four familiar classes of reptiles, amphibians, birds, and mammals. The British anatomist Thomas Henry Huxley made Latreille's definition popular and, together with Richard Owen, expanded Reptilia to include the various fossil "antediluvian monsters", including dinosaurs and the mammal-like (synapsid) Dicynodon he helped describe. This was not the only possible classification scheme: In the Hunterian lectures delivered at the Royal College of Surgeons in 1863, Huxley grouped the vertebrates into mammals, sauroids, and ichthyoids (the latter containing the fishes and amphibians). He subsequently proposed the names of Sauropsida and Ichthyopsida for the latter two groups. In 1866, Haeckel demonstrated that vertebrates could be divided based on their reproductive strategies, and that reptiles, birds, and mammals were united by the amniotic egg.
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