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Gruber, A. L., 1971, Problems of sexual dimorphism, population structure and taxonomy of the Ordovician genus Tetradella (Ostracoda): Journal of Paleontology, v. 45, p. 6-22.
Jefferies, R. P. S., 1973, The Ordovician fossil Lagynocystis pyramidalis (Barrande) and the ancestry of amphioxus: Philosophical Transactions of the Royal Society, London B, v. 265, p. 409-469.
Whittington, H. B., and Evitt, W. R. I. I., 1953, Silicified Middle Ordovician trilobites. Geological Society of America Memoir 59.
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