below the gill tracts. These areas are known as Kiemenplatten (or gill-tracts, though the former term is preferred). They are unique to eurypterids. Eurypterus are sexually dimorphic. On the bottom side of the first two segments of the mesosoma are central appendages used for reproduction. In females, they are long and narrow. In the males they are very short. A minority of authors, however, assume the reverse: longer genital appendage for males, shorter for females. The exoskeleton of Eurypterus is often covered with small outgrowths known as ornamentation. They include pustules (small protrusions), scales, and striations. They vary by species and are used for identification. For more detailed diagnostic descriptions of each species under Eurypterus, see sections below. Classification The genus Eurypterus belongs to the family Eurypteridae. They are classified under the superfamily Eurypteroidea, suborder Eurypterina, order Eurypterida, and the subphylum Chelicerata. Until recently, eurypterids were thought to belong to the class Merostomata along with order Xiphosura. It is now believed that eurypterids are a sister group to Arachnida, closer to scorpions and spiders than to horseshoe crabs. Eurypterus was the first recognized taxon of eurypterids and is the most common. As a consequence, nearly every remotely similar eurypterid in the 19th century was classified under the genus (except for the distinctive members of the family Pterygotidae and Stylonuridae). The genus was eventually split into several genera as the science of taxonomy developed. In 1958, several species distinguishable by closer placed eyes and spines on their swimming legs were split off into the separate genus Erieopterus by Erik Kjellesvig-Waering. Another split was proposed by Leif Størmer in 1973 when he reclassified some Eurypterus to Baltoeurypterus based on the size of some of the last segments of their swimming legs. O. Erik Tetlie in 2006 deemed these differences too insignificant to justify a separate genus. He merged Baltoeurypterus back into Eurypterus. It is now believed that the minor variations described by Størmer are simply the differences found in adults and juveniles within a species. The genus Eurypterus derives from E. minor, the oldest known species from the Llandovery of Scotland. E. minor is believed to have diverged from Dolichopterus macrocheirus sometime in the Llandovery. The following is the phylogenetic tree of Eurypterus based on phylogenetic studies by O. Erik Tetlie in 2006. Some species are not represented. Species Species belonging to the genus, their diagnostic descriptions, synonyms (if present), and distribution