J. Linn. Soc. Bot. 8: 44. 1864. -- Dichelyma falcatum (Hedw.) Myr. var. uncinatum (Mitt.) Lawton, Moss Fl. Pac. N. W. 231. 1973.
Because of its falcate, very narrow leaves, it is impossible to confuse Dichelyma uncinatum with any other member of the Fontinalaceae. It might be confused in the field with Dicranum, but no California member of that genus has almost semi-circular falcate leaves which are not distorted when dry.
Lawton (1971) asserted that, relative to Dichelyma falcatum, D. uncinatum is inconsistent in the two major discriminating characters, costa length and relative leaf width. Crum &
Anderson (1981) added the observation that D. falcatum also differs from the latter species in degree of leaf keeling, and in degree of leaf ranking. Aquatic and semi-aquatic mosses are notoriously variable, especially in costa development, and in the degree of prolongation of the leaf apices. Leaf ranking and degree of keeling are usually more reliable features in such mosses, and I have found these differences convincing in my own comparison of our western North American material with European and eastern North American material of D. falcatum.
Dichelyma uncinatum is one of that large number of mosses which grow primarily in humid west coastal North America and reappear in rare outliers in the northern Great Lakes region of eastern North America. Thanks to the careful collecting of Dr. Ruprecht Duell of Duisburg, West Germany, D. uncinatum is now recorded from California: on poison oak in overflow zone of small silt-laden creek immediately north of Willits, Mendocino County.