The occurrence of Middle Jurassic rocks in the Western Interior region of the United States is shown by the presence of the Bajocian ammonites, Stemmatoceras and Defonticeras, in the Twin Creek limestone of southeastern Idaho. The occurrence of Middle Jurassic rocks of the Bathonian stage is indicated by the presence of the ammonites, Arctocephalites and Cranocephalites, in the lower part of the Ellis formation of Montana. Marine Jurassic deposits older than the Bathonian appear to have been limited to the central part of the sedimentary trough and will probably be found only along the border area of Idaho and Wyoming, in north-central Utah, and in southwestern Montana.

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