At first glance, Interrupted Fern (Osmunda claytoniana) appears much like Cinnamon Fern. Both are typically tall, have twice-pinnate fronds* and grow in circular clumps often in moist soils. But whereas Cinnamon Fern has non-leafy, spore-bearing fronds that grow in the center of clusters, Interrupted Fern produces spores mid-frond, between green pinnae (for photos of these brown interruptions, visit Go Botany).
In the absence of fertile fronds, check the undersides of the pinnae where they meet the stalk: Cinnamon Fern (above) has small tufts of fuzz near this junction, which Interrupted Fern (below) lacks.
*Ferns typically feature fronds that are pinnate, twice-pinnate, or thrice-pinnate. In the case of both Interrupted and Cinnamon Fern, fronds are twice-pinnate, meaning each frond is divided into primary leaf segments (called pinnae) which are further divided into leaflets (called pinnules).