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 trichomatic hyphae.
Apothecia orbicular and revolute, spores 4- to
8-celled 6. P. polydactyla
Apothecia usually transversely oblong, spores
4-celled 7. P. horizontalis


1. Peltigera aphthosa (L.) Willd. Fl. Berol. Prodr. 347. 1787.

_Lichen aphthosus_ L. Sp. Pl. 1148. 1753.

Thallus closely attached to the substratum at the center, the lobes
ascending, 6 to 8 cm. in diameter, smooth and devoid of trichomatic
hyphae above, cephalodia more or less rounded and irregularly scattered
over the surface (Fig. 3), the lobes broad and rounded with crenate
margins, brown above, the lower surface having numerous veins, these
forming a dark brown nap at the center, the veins distinct and light
toward the margin, dark rhizoids extending from the veins; medulla
composed of thick-walled, densely interwoven hyphae, irregularly
disposed; apothecia on extended lobules, orbicular and frequently
revolute, the disk red-brown, 2 to 5 mm. in diameter, the margin entire
or crenulate; hypothecium pale brown; hymenium hyaline to pale brown
above; asci cylindrico-clavate; spores acicular, straight, 4- to
6-celled, 47 to 66 mic. long and 4 to 5 mic. wide (Fig. 8, d).