The Carboniferous eurypterid Eurypterus dumonti STAINIER, 1917 from Mechelen-aan-de-Maas (Maasmechelen), Belgium is redescribed and assigned to the genus Adelophthalmus . It is diagnosed as having a raised triangle of unknown function, dorsally on opisthosomal segment 7 and a carapace articulating laterally against the second opisthosomal segment. Earlier assignments of this species to the genus Unionopterus , based on the carapace shape figured in the original description (STAINIER 1917), are incorrect and the existence of a Carboniferous eurypterid with the characteristics described for Unionopterus must be questioned. The appendages in A. dumonti are completely exposed and provide an unrivalled insight into the number of appendage podomeres in the genus; this species is interpreted to have a podomere count consistent with most other eurypterids. Small pustules previously thought to be cuticle sculpture are here interpreted as diagenetic "dumb-bells" (see BRIGGS & WILBY 1996) following microbial activity on the carcase prior to fossilisation. These "dumb-bells" are the oldest non-marine record of this diagenetic feature. A number of other features are also interpreted differently from the original description. Possible lineages within the Adelophthalmus clade are identified, A. dumonti is probably closely related to A. imhofi (Czech Republic) and A. moyseyi (United Kingdom).