The trilobite family Illaenidae from the Ordovician and Silurian strata of the east Baltic, Girvan (Scotland), Anticosti island (Canada), and Kallholm (Sweden) is considered. "Knowledge of the doublure of the illaenid pygidium in certain instances is essential for specific determination.... The differences in the form of the anterior margin of the doublure may be related to variations in the form and strength of the axis of the pygidium."

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