Peltigera aphthosa, (L.) Hoffm. and P. polydactyla, (Neck.) Hoffm., are quite similar in general appearance to P. canina, (L.) Hoffm. P. aphthosa however, has the thallus smooth above, more or less sprinkled over with brown warts, and contains green algae, not blue-green as in the two following species. P. polydactyla differs from canina in being smooth above and nearly naked beneath and conspicuously reticulated with brown veins.