A lower Carboniferous limulacean Rolfeia fouldenensis gen. et sp. nov. is described. It is regarded as a sister group of Paleolimulus and distinguished from it by the presence of fixed opisthosomal spines and by the more primitive outline of the opisthosoma. Both characters could have been derived from belinuroid ancestors, and both were retained in the advanced belinurids and euproopids. The view that limulaceans are more closely related to certain primitive belinuraceans rather than descended from the euproopaceans is thus supported by the new evidence. Although Rolfeia was probably still capable of coaption, it is postulated that, like other limulaceans, it had adopted burial as a form of defence. It is suggested that the primary adaptive function of the fixed opisthosomal spines was to support the body over a soft substrate although they may also have served a defensive role.

The occurrence at Foulden of the eurypterid Cyrtoctenus peachi Størmer and Waterston is confirmed. The eoscorpiid Trachyscorpio squarrosus Kjellesvig-Waering 1984 has been described from Foulden and disjecta membra of Gigantoscorpio cf. willsi Størmer discovered in the recent excavation are described. The type of preservation and adaptive features of the chelicerates would suggest that the eurypterid and scorpion fragments are allochthonous and the xiphosurans probably autochthonous.

The prosomal morphology of Baltoeurypterus tetragonophthalmus (Fischer) from the Baltic Silurian is redescribed and reconstructed. The first eurypterid labrum and new secondary sexual characters of Baltoeurypterus are described. The radially-arranged coxae of Baltoeurypterus were capable of adduction and abduction for food mastication, but not promotor-remotor movements for locomotion. Joint diagrams are presented for the first time for an extinct arthropod. Promotion and remotion of the limbs occurred about subvertical trochanteral pivots, as in all other chelicerates except xiphosurans. Baltoeurypterus probably walked in a “slow” gait; a method of choosing possible gaits for extinct arthropods is outlined. Swimming in Baltoeurypterus was effected by means of a rowing action of the posterior limb pair, which is provided with complex joints for collapsing the paddle during the recovery stroke. The limb arrangement and joint mechanisms of Baltoeurypterus are intermediate between those of the xiphosurans and the arachnids. It is possible that a sister relationship exists between the eurypterids and some arachnid groups, which would render Merostomata and Arachnida unnatural assemblages.