The thallus of Peltigera rufescens has a cladomian structure. It is developed from axial filaments which are lodged in the veins and from which are developed several, short, ventral pleuridia and numerous well-developed, which are developed, dorsal ramifications.
The latter form the remainder of the thallus.
The ascocarps develop directly from a small number of marginal, dorsal pleuridia which form a well-delimited primordium from which is built up a primary corpus, similar to the type formed in graphidian and lecanorian species. To this is added a parathecial apparatus. Although this development is to be qualified lecanorian, certain peculiarities may be noted; these are a very long angiocarpic stadium in the course of which are built up a parathecioid envelope and a marginal crown, the characters of which are intermediate between those of primary and secondary elements.
The asci are bitunicate and ‘archaeascé’.