This volume 154 offers the first comprehensive examination of the genus Syntrichia in the Holarctic, covering a diverse range of species found on all continents. The focus is primarily on North American species, which are often poorly understood, while also considering the connected Eurasian species of the extra-tropical Northern Hemisphere. Approximately 110 taxa are analyzed, leading to the identification of 34 species, including 59 taxa of Syntrichia, alongside three species reclassified into other genera. Notably, sixteen of these taxa are newly described, with five new species—four endemic to the Americas and one also found in Eurasia. Additional new taxa are identified, with some restricted to North America and others to Eurasia. Three names previously considered synonyms are now recognized as valid species, while twenty-four others are designated as new synonyms. The work includes detailed descriptions, an identification key, and several new characteristics that may aid in classifying Syntrichia. This resource is invaluable for bryologists studying North American species and offers significant support for identifying all Syntrichia taxa in the Holarctic. It also lays the groundwork for future molecular-genetic research on Syntrichia and related genera.
