Sagenopteris williamsii is an extinct pteridosperm (seed fern) that is known from the late Jurassic and Early Cretaceous strata of the western interior of North America.

Newberry in 1891, based on specimens from the Great Falls coal field in Montana.

Bell based on additional specimens from western Canada.

They had a petiole, anastomosing venation, and a midvein that thinned toward the apex.

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