Thallus foliose, with plectenchymatous cortex above (Fig. 5), or both above and below (Fig. 2), with medulla of loosely interwoven hyphae, trichomatic hyphae, usually present, attached to the substratum by compound rhizoids; apothecia of considerable size, commonly on extended lobes, usually imbedded in the tissues on the upper side, or more rarely on the lower side; exciple inconspicuous; hypothecium usually light or darker brown; hymenium usually pale below and brown or tinged brown above; paraphyses simple or branched, distinct, seldom gelatinized or coherent; spores hyaline or brown, 4- to several-celled, elongated. KEY TO THE GENERA 1. Cortex developed on the upper side of thallus only, spores hyaline Peltigera. 2. Cortex developed on both upper and lower sides of thallus, spores brown Nephroma. _Peltigera Willd. Fl. Berol. Prodr. 347. 1787._ Thallus foliose, usually adnate toward the center, with the lobes more or less ascending at the margins, green-gray varying toward brown, the upper surface sometimes bare, or again clothed with trichomatic hyphae, giving it a downy appearance, or bearing cephalodia or isidioid branchlets, the lower surface usually conspicuously veined, with tufted rhizoids descending from the veins, color of these light or dark; cross section showing two distinct layers, the upper plectenchymatous cortex composed of 2 to 4 layers of meshes, and the medulla, composed of densely interwoven and irregularly disposed hyphae; lower cortex lacking, but the hyphae of the lower portion in some instances more or less horizontally arranged and produced into hyphal rhizoids, thus serving for support and protection much like a true plectenchymatous cortex; apothecia usually orbicular, frequently revolute, imbedded in the upper surface of the lobes; exciple plectenchymatous (Fig. 4); hypothecium of interwoven hyphae, usually tinged brown; hymenium commonly pale below and brown above; paraphyses usually simple, but some branched ones present in all of the species, hyaline in the main, but usually enlarged and tinged brown at the apex; asci usually cylindrico-clavate; spores hyaline, fusiform to acicular, sometimes curved, 4 to 8-celled, 8 arranged parallel in the asci. The algal host cells lie in the medulla, just below the upper cortex. KEY TO THE SPECIES OF PELTIGERA