These ancient amphibians illustrate various stages in the evolution of the backbone of mod ern Vertebrata from the notochord or seg mented rod of cartilage from which it was derived. In the smaller and more primitive types the segments of cartilage are but slightly ossified in a number of separate plates or in complete rings of bones. In others each verte bra is composed of two or four pieces, which re main separate during life instead of consolidat ing into a single bone, as in modern vertebrates. In others, again, the vertebra is completely united. The oldest known labyrinthodonts are from the Carboniferous rocks and are related to some of the older Palaeozoic dipnoan fishes, from which they may have been descended. In the Permian they attained large size and great abundance, and continued into the Triassic period, by the end of which they had become extinct. Their foot-tracks, often preserved in muddy sandstones of these periods, are some times curiously like the impressions of a human hand, whence they received the name of Chesro therium, or before their nature was recognized. The frogs and sala nianders are probably descended from primitive labyrinthodonts, but are very little known as fossils. One fossil species, allied to the giant
salamander of Japan, was found in the Miocene strata of Oeningen many years ago (1726) and was supposed by an early naturalist to be the fossil skeleton of a man, and described as c'homo diluvii testis et theoscopoe— the man who was witness to the Deluge and saw God— a quaint reminder of the geological speculations of two centuries ago. For breeding habits, dis tinctive characteristics and modes of life see special articles under the various species. For good recent treatment of the Amphibians consult Gadow, 'Amphibia and Reptiles' (London 1901); Boulenger, 'Catalogue of Eatrachia' in British Museum (London 1882) ; and many papers in English scientific periodi cals: Cope, 'Batrachia of North (Bulletin 34, United States National Museum, Washington 1889), which contains an extensive bibliography; also 'Amphibia,) by Lyddeker, Cunningham, Boulenger and Thompson (London 1912). Dickerson's 'The Frog Book' (New York 1916) gives a popular account of the group.