Agnostid trilobites from the Upper Ordovician of Sweden and Bomholm, Denmark

Abstract

Arthrorhachis tarda (Barrande, 1846) and Sphaeragnostus cingulatus (Olin, 1906) are described from the Upper Ordovician (Harjuan Series) of Sweden and Bornholm, Denmark. S. cingulatus is extremely rare, while A. tarda is generally abundant in middle Ashgillian strata and known from a great number of localities in southern and south-central Scandinavia. Outside Scandinavia, A. tarda is frequently encoun- tered in the pre-Himantian Ashgill of Poland, Bohemia, various parts of the British Isles, and Kazakh­stan. In most of these areas, the species is generally confined to mudstone sequences, and it is usually associated with faunas of Mediterranean type. The generic concept of Sphaeragnostus is revised.