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Trigonia
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Trigonioida
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Trigonia

bivalve
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Encyclopaedia Britannica's editors oversee subject areas in which they have extensive knowledge, whether from years of experience gained by working on that content or via study for an advanced degree....
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History

Trigonia, genus of mollusks that first appeared during the Jurassic period, which began about 208 million years ago. The still-extant Trigonia has a triangular shell with distinctive concentric ridges on its surface as well as nodular outgrowths. A different ornamental pattern is present in the posterior parts of the shell.