Catillaria typically grows as a thin, crust-like layer (
crustose) on various surfaces. The fungal body (
thallus) can appear in several forms – it may be barely visible, cracked, warty, or divided into small polygonal areas called . These lichens display various colours including white, grey, green, brown, or black, though some species that grow on other lichens may lack a visible thallus entirely. Like all lichens,
Catillaria represents a
symbiotic partnership between a fungus and
photosynthetic algae (known as the ). In this genus, the algal partners are
green algae, specifically from genera such as
Dictyochloropsis,
Myrmecia, or
Trebouxia. The fungal portion lacks a protective outer layer () or has only a rudimentary one. The reproductive structures (
apothecia) are black and typically lack a powdery coating (). They have a distinctive microscopic structure, including specialised cells called
paraphyses that have abruptly swollen tips capped with dark brown pigmentation. The spore-producing cells (
asci) typically contain eight spores, though occasionally up to 16, and show a characteristic blue reaction when treated with
iodine-based
stains. The spores themselves are colourless and divided into two cells by a single wall (
septum), without any surrounding gelatinous sheath. The genus can be distinguished from similar lichens by its combination of asci that turn blue with iodine, paraphyses with dark-capped swollen tips, two-celled spores without a gelatinous coating, and spore-producing cells arranged in chains. While the similar genus
Halecania shares many of these features, it differs in having thick-walled spores with a distinct gelatinous coating and is not closely related based on genetic analysis. When analyzed chemically using
thin-layer chromatography, these lichens do not show evidence of specialised
lichen products. ==Species==