N. natashae, a new species in the Dumontiaceae (Cryptonemiales), is described from Alaska [USA]. It differs from its congeners in its brownish-red color, its thin blades, the presence of highly refractive stellate medullary cells, the formation of a continuous layer of tetrasporangia in the cortex, relatively long lateral branches on the carpogonial and auxiliary cell branches, no gonimoblast fusion cell, and the appearance of mature cystocarps first near the base of the blade. A comparison of post-fertilization stages in this species with published accounts of other species in the Dumontiaceae shows that the family as a whole can be characterized by the division of the carpogonium following fertilization, initiation of primary connecting filaments from each carpogonial derivative cell usually after each has fused with another cell of the carpogonial branch, and the connecting filament dividing near an auxiliary cell with the distal cell growing onward and the proximal cell fusing with the auxiliary cell and itself producing the gonimoblast filaments. Significant features of this characterization are at odds with those currently accepted for the family.