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It is developed less generally than some others, but occurs conspicuously in England and Germany. The magnesia limestone is also remarkable as the last rock in which appears the leptaena, or producta, a conchifer of numerous species which makes a conspicuous appearance in all previous seas.
Hippopodium ponderosum, Sowerby. 1/4 diameter. Leptaena Moorei, Davidson. No less than eight or nine species of Spiriferina are enumerated by Mr. Davidson as belonging to the Lias. Palliobranchiate mollusca predominate greatly in strata older than the Trias; but, so far as we yet know, they did not survive the Liassic epoch. Ammonites Bucklandi, Sowerby. Ammonites bisulcatus, Brug.
The same species has sometimes continued to exist under slightly modified forms throughout the whole of the Ordovician and Silurian as well as the entire Devonian and Carboniferous periods, as in the case of the shell generally known as Leptaena rhomboidalis, Wahlenberg. No less than fifteen commonly received species are demonstrated by Mr.
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