The type species of the Ordovician agnostid trilobite Geragnostus Howell, 1935

The type of the widespread Ordovician agnostid trilobite Geragnostus, G. sidenbladhi (LINNARSSON, 1869) from the upper Tremadocian Ceratopyge Limestone of Sweden, is redescribed and a lectotype is selected. Geragnostus HOWELL, 1935 bears a strong similarity to Arthrorhachis HAWLE & CORDA, 1847. Obviously the two genera are closely allied, and Geragnostus may eventually be regarded as a junior subjective synonym of Arthrorhachis. It is suggested, however, that the distinction between Geragnostus and Arthrorhachis might be upheld if the latter genus is taken to include metagnostids in which the posterior lobe (M3) of the pygidial rhachis is distinctly shorter than the length (sag.) of the anterior and middle lobes combined (M1 + M2)