Yarrumia forms large, leaf-like that can blanket bark or moss in patches up to 20 cm across—and occasionally twice that size. Although these lobes often elongate into narrow, beard-like strands that dangle from their
substrate, each strand still has a distinct upper and lower surface (i.e., it is ), so anatomically the thallus remains foliose; accordingly, the
New Zealand Plant Conservation Network lists
Yarrumia colensoi among the foliose lichens, and Galloway notes the thallus as "orbicular to spreading, often ± pendulous". The upper surface is smooth to faintly wrinkled, whilst the underside carries a sparse layer of pale yellow hairs (a
tomentum) broken by tiny yellow pores called
pseudocyphellae. These pores act as minute air vents, helping the lichen to breathe by allowing
gas exchange through the otherwise waterproof . Inside, the supporting fungal tissue (the
medulla) is yellow to orange-red, a pigment suite that immediately separates the genus from most of its relatives. The partnership that powers the lichen is a
green alga housed in rounded cells measuring 3–7
μm wide. In addition,
Yarrumia harbours scattered internal
cephalodia—dark, knot-like packets that hold
cyanobacteria capable of
fixing atmospheric nitrogen, thus enriching the lichen's diet. Reproduction is by conspicuous, stalked (pedicellate apothecia) that rise from the lobes like tiny buttons. Each disc is rimmed by a sheath of thallus tissue (the ) and sits above a pale yellow-brown hymenial base (). The
ascospores produced within are
ellipsoid, with rounded or slightly pointed tips; they usually develop one to three cross-walls (
septa) and measure roughly 30–38 × 9–11 μm, although smaller or larger extremes occur. Chemically the genus is distinguished by
polyporic acid—first detected in these species in the 1950s—and a suite of unusual stictane
triterpenoids. ==Species==