Abstract
Fossil phycomata of prasinophycean green algae are here described for the first time from the Silurian Bainbridge Formation of southeastern Missouri. Two new species, Melikeriopalla fistulosa and Pterospermella scruposa, are described. Four new combinations, Cymatiosphaera retieulosa (Kir'yanov, 1978), Dietyotidium coaretatum (Kir'yanov, 1978), Dictyotidium venulosum (Playford, 1981), and Dictyotidium cataphractum (Martin, 1978) are also made. The genus Melikeriopalla Tappan & Loeblich, 1971 is revised. A late Pridolian age for the upper part of the Bainbridge Formation is indicated by the prasinophycean flora. A comparison of fossil and recent phycomata illustrates the extreme evolutionary conservatism of the phycomaphase of the prasinophycean life-cycle. Fossil evidence suggests that the modern genus Pterosperma Pouchet, 1893 can be divided into two or more genera based on phycoma morphology.

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