There are 13 species of Fissidens known or expected to occur in Delaware. A key to these species is given along with the following information for each species: short description; comments on ecology; distribution in Delaware; total range distribution; and citations to illustrations in selected North American floras. The two most credible theories for the origin of the odd Fissidens leaf—enation vs. axis reorientation—are noted. Evidence is given in support of the theory that this odd leaf morphology is due to the reorientation of the mitotic spindle of the leaf apical cells. A possible cause for this reorientation—the similarity between the two-sided stem apical cell and the two-sided leaf apical cells—may have resulted in the formation of a leaf lamina in the plane of the apical cell of the stem. As a result the entire stem can be envisioned as though it were a single leaf. Important morphological features of Fissidens are described and particular attention given to the variations exhibited by unequal vaginant laminae.