Literally, ‘double breathing’. Often ranked as a subclass, the group includes the extant lung-fish and their fossil relatives (e.g. the Middle Devonian Dipterus and the Triassic Ceratodus). Early forms have an elongated body, a well-ossified internal skeleton, heterocercal tail, fleshy-lobed fins, and cosmoid scales. Teeth are absent, but one of the commonest fossils is the broad fan-shaped tooth plate that served for shearing and crushing small invertebrates. Dipnoans first appear in Lower Devonian rocks and were common in freshwater habitats in the late Palaeozoic and the Triassic; thereafter their fossil remains are very sparse. There are just three surviving genera (Neoceratodus, Protopterus, and Lepidosiren), all tropical.