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From The Collaborative International Dictionary of English v.0.48:
Jurassic \Ju*ras"sic\, a. (Geol.) Of the age of the middle Mesozoic, about 190 to 140 million years ago, including, as divided in England and Europe, the Lias, O["o]lite, and Wealden; -- named from certain rocks of the Jura mountains. It was noted for the predominance of dinosaurs on land, and the development of the first birds and mammals. -- n. The Jurassic period or formation; -- called also the Jura. [1913 Webster +PJC]