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In 1865, several species belonging to three different genera of the same family of perennibranchiate Batrachians were found in the coal-field of Kilkenny in bituminous shale at the junction of the coal with the underlying Stigmaria- bearing clay.
They were of the Labyrinthodont family, and their association with the fish of the coal, of which so large a proportion are ganoids, reminds us that the living perennibranchiate amphibia of America frequent the same rivers as the ganoid Lepidostei or bony pikes. Slab of sandstone from the coal-measures of Pennsylvania, with footprints of air-breathing reptile and casts of cracks.
It is evident, therefore, that the Mexican Axolotls, although they have been perennibranchiate for a great number of generations, have not lost the hereditary tendency to the metamorphosis which changes the larvae of Amblystoma elsewhere into an air-breathing terrestrial animal.
After comparing the Salamanders and the perennibranchiate Urodela with the Tadpoles and the Frogs, and enunciating the law that the more highly any animal is organised the more quickly does it pass through the lower stages, Meckel goes on to say
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