Ascomata: perithecioid pseudothecia, 150-200 µm in diam., subglobose, ovoid or obpyriform, dark brown or black, ostiolate, immersed, semi-immersed or subsessile, scattered or in small groups, sometimes confluent in necrotic patches or in gall-like deformations of the host thallus ascomatal wall: entirely brown to dark brown, pseudoparenchymatous, 30-40 µm thick in section, composed of several layers of isodiametric to tangentially elongated cells paraphysoids: 1.5-2.5 µm in diam., filiform, hyaline, septate, abundantly branched and anastomosed periphyses: ostiolar filaments clearly differentiated, branched, hyaline, green pigmented at the top of the ostiolar region asci: clavate to subcylindrical, 60-90 x 12-18 µm, 4-8-spored ascospores: brown or dark brown the oldest, with the tips of the end cells often pale brown to subhyaline, ellipsoid to broadly fusiform, often slightly curved, rounded at the apices, usually 3-septate, rarely 2- or 4- septate, ±constricted at the septa, 20-25(-30) x 7-10 µm. Hosts: Pyrenidium actinellun s. l. is known from a wide range of hosts, including crustose and foliose lichens on various types of substrates; in the Sonora Desert region it is known from the thallus of Caloplaca sp., Phaeophyscia sp., Teloschistes chrysophtalmus and Trapeliopsis granulosa World distribution: probably cosmopolitan, widely distributed in Europe and recorded from Asia, Greenland, North and South America, Africa and Australia Sonoran distribution: collected from a few localities in Arizona, Sinaloa and Baja California Sur. Notes: Pyrenidium actinellun s. l. constitutes a group of species that differ in the form and size of ascospores, size and characteristics of the ascomata, and nature of the host. In a strict sense P. actinellun refers to a lichenicolous fungi that grows on Leptogium teretiusculum, and it is characterized by the sessile ascomata growing on the host thallus. Hawksworth (1980, as Dacampiosphaeria rivana, & 1983) includes numerous synonyms in a broad concept of this species. The revision of this genus is being undertaken by the authors.