ed.

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

Upper surface of the thallus bearing cephalodia 1. P. aphthosa
Upper surface of the thallus devoid of cephalodia.
Thallus bearing trichomatic hyphae above.
Upper surface bearing isidioid branchlets or lobules 2. P. praetextata
Upper surface devoid of isidioid branchlets or lobules.
Orbicular sorediate areas on the upper surface of
the thallus 3. P. sorediata
Soredia lacking on the upper surface.
Lower surface of the thallus of light color 4. P. canina
Lower surface partly or wholly dark 5. P. rufescens
Thallus devoid of tri