This paper shows the results of a research into the Devonian trilobites of the southern Cantabrian Mountains. The systematics of the most common group, the subfamily of the Asteropyginae, forms the major part of this work. More than 50 species, belonging to sixteen (sub)genera are distinguished; thirteen species are newly described and four species are redescribed. It turned out that many species recently described from Aragón, Asturias and the Armorican Massif also occur in the southern Cantabrian Mountains. The find of Metacanthina lavidensis n.sp. in the La Vid Fm. makes it probable that the genus Metacanthina descended from Pilletina in the Siegenian. The morphological features and age of Delocare? dalii n.sp. support the theory that Delocare descended from Paracryphaeus in the Siegenian. A first time a Neocalmonia species N. cantabrica n.sp.) has been found outside Afghanistan. Its morphological features and occurrence in the Huergas Fm. indicate it descended from the genus Bradocryphaeus in the lowermost Givetian. The genus Neocalmonia therefore originated in Europe. The find of G. (Greenops) ultimus n.sp. in the Portilla Fm. prolongs the known existence of the genus Greenops far into the Frasnian. Bradocryphaeus sexspiniferus n.sp. is the first species of Frasnian age with six pairs of lateral pygidial spines which has been found outside Afghanistan. Meraspides of Kayserops obsoletus are discussed. Apart from the Asteropyginae more than 60 other trilobite species are mentioned per stratigraphic level and for the greater part depicted. In the Devonian the trilobite faunas of the Asturo-Leonese and Palentian Basins show a gradually growing difference. In the Gedinnian and Siegenian the difference is not very marked. In the Emsian the Palentian Basin yields, beside common species with the Asturo-Leonese Basin, many elements which indicate a deeper water environment (Odontochile, Reedops, Cheirurus (Pilletopeltis), Xiphogonium and Astycoryphe). In the Middle and Upper Devonian the two basin
