Ateleaspis tessellata Traquair, a non-cornuate cephalaspid from the Upper Silurian of Scotland

Ateleaspis tessellata Traquair, a non-cornuate cephalaspid from the U. Silurian of Scotland, is re-examined on the evidence of new material from the original localities. The species is found to be less 'primitive' than was previously thought in that the pectoral fins are separated from the trunk posteriorly by a distinct sinus and the two dorsal fins are developed essentially as in Aceraspis robustus Kiaer. The ventral visceral exoskeleton is described for the first time. Ateleaspis and Aceraspis are so close in so many respects that retention of separate genera is unnecessary; Aceraspis must be suppressed since Ateleaspis has priority. Revised diagnoses of the genus Ateleaspis and its two species, A. tessellata Traquair and A. robustus (Kiaer), are given and their relationships with other non-cornuate cephalaspids discussed.