Pycnoporidium sinuosum, n. sp., is described from the Upper Cretaceous San Cristobal formation of Guatemala. The species is the first known Late Cretaceous representative of this Late Jurassic-Cretaceous genus. The authors discuss the classification of genera of the Solenoporaceae of the late Mesozoic and suggest that the genus Pycnoporidium be excluded from the family. A suggestion is made that the genus be placed among the Siphonocladaceae of the Chlorophyceae. A probably symbiotic alga, "Girvanella cfr. tosaensis Yabe et Toyama," occurs in association with P. sinuosum.

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