 "Herefordopterus" and "Hughmilleria" shared a subtriangular carapace outline with the pterygotids and the wide telson and genital appendages of all three taxa were similar, although the genital appendages of "Herefordopterus" and "Hughmilleria" were more similar to those of "Slimonia" by the division of these into three segments, in contrast to the undivided morphology in the pterygotids.Still, "Herefordopterus" and "Hughmilleria" differ from pterygotids and slimonids by the presence of paired spines on the walking appendages, that along with the characteristics that "Slimonia" and "Ciurcopterus" share, suggest that the hughmilleriids are more distant from the pterygotids than "Slimonia" is.Within Hughmilleriidae, both genera possessed a marginal rim much broader anteriorly than posteriorly and spiniferous appendages, but "Hughmilleria" had 18-20 gnathobasic teeth on appendage VI, unlike "Herefordopterus" and the pterygotids, which had 12-13.