Abstract
The discovery and study of three new species of Trimmatothelopsis from Southwestern North America, T.californica, T.mexicana, and T.novomexicana, adds not only to the diversity of the genus and family but generated new insights into the occurrence of two ascus types in the genus and the variety of conidiogenous cells and conidia. Trimmatothelopsis now includes 15 species with a mainly Holarctic distribution (Asia, Europe, North America) and one species in Australia. A key is supplied to the genus. An overview of the genus Trimmatothelopsis is given, including differentiation from other genera of Acarosporaceae. The monotypic genus Thelocarpella is considered to be a synonym of Trimmatothelopsis. The new combination Trimmatothelopsiswirthii is proposed. The ascus type is shown to be variable in the genus with species with two types being intermixed with each other in our phylogeny.