"The cephalic characters which the post-Cambrian trilobite families Encrinuridae, Pliomeridae, Cheiruridae, and Phacopidae have in common are discussed, together with others which illustrate the distinctness of the Phacopidae. The importance of growth-rate studies is emphasized, particularly with reference to the frontal lobe of the glabella. The Calymenidae share most of the features common to the Cheiruracea and Phacopacea, and are considered to be closely related to them, and the Odontopleuridae also share certain features. The possibly neotenous origin of the Cheiruracea and Phacopacea is discussed."

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