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Note: Do not rely on this information. It is very old.
Trinucleidae
Trinucleidae, a family of Trilobites (q.v.), in which the head sheild is greatly developed, and the two hinder angles of it are prolonged back as spines. The type-genus Trinucleus is most abundant in the Bala beds of North Wales, while the family is entirely restricted to the Silurian and Ordovician systems.