Hughmilleriidae (the name deriving from the type genus Hughmilleria, which is named in honor of Scottish geologist Hugh Miller) is a family of eurypterids, an extinct group of aquatic arthropods. The hughmilleriids were the most basal members of the superfamily Pterygotioidea, in contrast with the more derived (more "advanced") families Pterygotidae and Slimonidae. Despite their classification as pterygotioids, the hughmilleriids possessed several characteristics shared with other eurypterid groups, such as the lanceolate telson (the most posterior segment of the body). Hughmilleriids are defined as pterygotioid eurypterids with swimming legs similar to those of the type genus, Hughmilleria (that is, 7th and 8th leg segments narrow and 9th segment very small), and whose second to fifth pair of appendages were spiniferous. Some further diagnostic characters unite the group, such as the slightly enlarged chelicerae (frontal appendages) and the streamlined shape of their bodies. The family contains only two genera, Hughmilleria and Herefordopterus, though other genera have been referred to the family in the past, such as genera now considered part of families such as the Mycteroptidae and the Waeringopteridae. The hughmilleriids were the most basal group of pterygotioid eurypterids, lacking the derived features that would come to evolve in the Slimonidae and Pterygotidae, such as flattened and expanded telsons (the posteriormost segment of the body, this feature is shared by both derived families) and enlarged cheliceral claws (exclusive to the pterygotids). In spite of the great similarity of both genera, Herefordopterus had derived characteristics that suggest a close relationship with Slimonidae and Pterygotidae, such as the marginal ornamentation of the telson. On the other hand, Hughmilleria had certain resemblance to the basal adelophthalmid Eysyslopterus, sharing a triangular anterior carapace margin, possibly a plesiomorphic (of a common ancestor) trait. Description Hughmilleriid eurypterids ranged in size from 6 to 20 centimetres (2 to 8 inches), representing a group of relatively small eurypterids. Hughmilleriids would be dwarfed by some of their more derived (more "advanced") relatives within their superfamily, especially the pterygotids which would surpass lengths of 2 metres and become the largest known arthropods to ever live. Like all other chelicerates, and other arthropods in general, the hughmilleriid eurypterids possessed segmented bodies and jointed appendages (limbs) covered in a cuticle composed of proteins and chitin. The chelicerate body is divided into two tagmata (sections); the frontal prosoma (head) and posterior opisthosoma (abdomen). The appendages were attached to the prosoma, and were characterized in