1. RETICULARIA. Æthalium composed of numerous slender sinuous sporangia which repeatedly branch and anastomose. 2. CLATHROPTYCHIUM. Æthalium composed of numerous regular erect sporangia. _b. Sporangia simple._ 3. CRIBRARIA. Capillitium of slender threads combined into a network of polygonal meshes. 4. DICTYDIUM. Capillitium of numerous convergent ribs, which extend from base to apex, and are united by fine transverse fibers, thus forming a network of rectangular meshes. I. RETICULARIA, Bull. Æthalium composed of numerous slender sinuous sporangia, which repeatedly branch and anastomose, closely packed together and seated upon a common hypothallus, the apices of the final branches coherent at the surface, and naked or covered by an additional corticate layer. Walls of the sporangia consisting of a thin membrane, with abundant fibrous thickenings, presenting broad expansions, narrowing to thin flat bands, and reduced in many places to slender fibrous threads. Spores abundant, globose, umber or violaceous. After the maturity of the spores disintegration of the sporangial wall begins, the thin membrane disappearing more rapidly than the fibrous thickenings or the portions of the sporangial walls near the base, which are more compactly grown together; there is thus left at each stage an increasing number of the shreddy fibers mingled with the spores. 1. RETICULARIA SPLENDENS, Morg. n. sp. Æthalium pulvinate, circular or more or less elongated and irregular, seated on a conspicuous silvery hypothallus; the surface naked, bright umber, smooth and shining. Walls of the sporangia firm and quite persistent, pale umber, slowly disintegrating, consisting for the most part of wide expansions, with their angles tapering to narrow bands and slender threads. Spores in the mass pale umber, globose, most of the surface reticulate, 7-9 mic. in diameter. See Plate III, Fig. 10. Growing on old wood. Æthalium from 1 to several centimeters in extent and 5-10 mm. in thickness, usually growing singly, rarely close enough to be confluent. This species has lately been referred to _Reticularia rozeana_, Rost., but it varies greatly from the account given of that species in the Journal of Botany for September, 1891. 2. RETICULARIA UMBRINA, Fr. Æthalium pulvinate, roundish, more or less irregular, the surface covered by a thin, silvery, shining, common cortex, which at the base is confluent with the hypothallus. Walls of the sporangia umber or rusty-brown next the base, with broad expansions in places thickly grown together, toward the surface passing into narrow bands and abundant fibrous threads, which rapidly disintegrate. Spores in the mass umber or rusty brown, globose, most of the surface reticulate, 7-9 mic. in diameter.