Life habit: fungicolous, incl. lichenicolous, commensalistic or parasitic, often gall-forming, non-lichenized Basidiomata: mostly cerebriform to foliose, ranging from 0-2 mm to as much as 8 cm wide; distinct basidiomata lacking in species endoparasitic in other basidiomycetes or in hymenia of discomycetes; in the case of lichenicolous taxa often growing within galls induced in the host lichen hyphae: context hyphae: thin- to thick-walled; clamp connections: occurring in most species of the genus, but lacking in a few non-lichenicolous taxa, and not yet observed in some lichenicolous species haustorial branches: frequent, usually with clamp connections hymenium: containing numerous probasidia, sometimes intermixed with conidiogenous cells and/or hyphidia; probasidial initials: subspherical, ellipsoid or clavate, sometimes with an attenuated base; mature structures: globose to ellipsoid, pyriform, clavate, or sometimes stalked, capitate, proliferations occurring through the basal clamp; hyphidia: absent or reduced, but in a few species forming a distinct hymenial layer; cystidia: absent basidia: when mature, (1-)2-4-celled, with longitudinal, oblique, or transverse septa epibasidia: subcylindrical, elongate basidiospores: subspherical to ellipsoid, with a distinct apiculus, exceptionally almost limoniform or gasteroid Anamorph: often present conidia: single or catenate asteroconidia: frequent in lichenicolous species Geography: cosmopolitan Substrate: always associated to fungi: mycelia, ascomata, basidiomata, apothecia or thalli of lichens, often intrahymenial. Notes: Of the species treated, one is not yet known from the greater Sonoran Desert region, but is found in adjacent areas of California, and hence is likely to occur also in the flora area. Several additional undescribed Tremella species are known from the Sonoran area, but will be described later. The most common and remarkable of them develops over Usnea thalli in Arizona and can easily be mistaken for Biatoropsis usnearum or Cystobasidium usneicola.