Based upon our study of type specimens, we confirm that Anthoceros phymatodes M. Howe from California is synonymous with Phaeoceros bulbiculosus (Brot.) Prosk. from Portugal. Furthermore, this taxon forms the basis for the recently named, monotypic genus Phymatoceros Stotler, W. T. Doyle & Crand.-Stotl. Our observations of the morphology, anatomy, and phenology of living populations from both California and Portugal reveal a suite of characters that discriminate this taxon, not only from Phaeoceros Prosk., but also from all other hornwort genera. These include rounded to spindle-shaped chloroplasts with abundant, bulging starch grains that may obscure a pyrenoid; highly dimorphic, dioicous thalli; a single antheridium per antheridial chamber, rather than 2 to 4 as in Phaeoceros; and spores that are fuscous at maturity.