Nash, T.H., Ryan, B.D., Gries, C., Bungartz, F., (eds.) 2002. Lichen Flora of the Greater Sonoran Desert Region. Vol 1.
Notes: The second species, P. lepidotella (Nyl.) Gotth. Schneid., is apparently known only from the type collection. This specimen is in a poor condition, and could not be examined in detail by Schneider (1979), and consequently the generic description encompassing both species is not possible. The description given below is based on the type species, and it would be repetitious to have a separate generic description.