Frullaniaincumbens is morphologically similar to other species of subg. Australes, which include F. anomala Hodgs. and F. media (Hodgs.) Hatt. of New Zealand, and F. fugax (Tayl.) Tayl. from New Zealand and Australia. All taxa of subg. Australes share the following features: a campanulate lobule that is generally at least twice as long as wide and parallel or incumbent to the stem; lobule with wide-rounded apices and almost always constricted above the mouth; and the leaf-lobes with rounded to obtuse apices. Generally, Frullania incumbens can be distinguished from these species by the incumbent position of the lobules in relation to the stem. When fertile material with perianths is present, F. incumbens can immediately be differentiated from F. anomala and F. media. The perianths of F. incumbens are typically strongly 3-keeled and have a smooth surface. The perianths of F. anomala are also 3-keeled, but densely covered with multicellular projections, and the perianths of F. media also have a smooth surface, but are typically plicate with several keels.
Frullania fugax can be separated from F.incumbens by the following suite of features: plants of F. fugax are typically smaller, the lobules are narrower, to 2.5Χ long as wide and usually lie more or less parallel to the stem, and the lobes are often gemmiferous or caducous. The closest affinities of F. incumbens may lie with F. subincumbens Hatt. from Lord Howe Island, a species of subg. Australes where the incumbent form of the lobule is also present. Hattori (1987b) provided a suite of characters that distinguished F. subincumbens from F. incumbens, including the frequent presence of squarrose leaves and the scattered tuberculae on the perianth for F. subincumbens; 4-keeled perianth versus 3-keeled; short-beak versus long-beak; and only the dorsal lobe appendiculate versus appendiculate at both dorsal and ventral lobes. However, based on New Zealand specimens, F. incumbens shows morphological intergradation in some of the characters Hattori used to separate F. subincumbens from F. incumbens. Frullania subincumbens is only known from the holotype and more collections are required in order to confirm its taxonomic status and establish its relationship with other species of subg. Australes.
Geographic distribution:
Apparently endemic to Lord Howe Island (Fig. 7.1).