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Trilobites and brachiopods are described and figured from the Caradoc Series of the Twll Ddu district, northeast of Snowdon. They indicate a Longvillian age for the Banded Pumice Tuffs of the Bedded Pyroclastic Series, and exhibit affinities with corresponding faunas in South Shropshire as well as other parts of North Wales and Northern England.
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W T Dean (1965) studied this question.
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