Further species of what we consider Stylonuracea today were established in 1859, both in Huxley and Salter's great monograph and in Hall's equally important work. Laurie, in 1892, added the new genus Drepanopterus from a large suite of stylonuroids from the Silurian Gala-Tarranon beds of Scotland. It was not until the celebrated monograph of Clarke and Ruedemann in 1912 that a serious attempt was made to subdivide the "stylonurids" into several generic and subgeneric categories. Dolichopterus was recognized as a genus and the "true stylonurids" (those without paddles) were divided into four definite groups, which were considered subgenera: (1) Stylonurus (Stylonurus), (2) Stylonurus (Tarsopterus), (3) Stylonurus (Ctenopterus), (4) Stylonurus (Drepanopterus) and a fifth group which they considered "Indeterminate species."
Clarke and Ruedemann, however, still considered Dolichopterus and Stylonurus as part of the family Eurypteridae. Diener in 1924 recognized the obvious and major differences between Stylonurus and its subgenera on the one hand and members of the family Eurypteridae on the other, and erected the new family Stylonuridae Diener.
In 1951 Størmer, in an important paper, established the family Rhenopteridae on the significant basis of the ventral shield of the carapace and the form of the metastoma. In all outward appearance—that is, dorsally—the Rhenopteridae could well have been mistaken for Stylonuridae were it not for the very different underside. As in scorpions, considerable and increasing evidence is being accumulated, indicating that the major morphological differences of taxonomic importance are to be found on the ventral surface. This applies not only to higher taxa but to species and subspecies.
Several other families were added to what became known as the superfamily Stylonuracea Diener, 1924 (nom. transl. Størmer, 1951 [ex Stylonidae, 1924]). Størmer gave new names to some of Clarke and Ruedemann's 1912 genera which were junior homonyms, among them changing Tarsopterus to Tarsopterella. Later, other families were added to the Stylonuracea, namely, Dolichopteridae Kjellesvig-Waering and Størmer, 1952; Hibbertopteridae Kjellesvig-Waering, 1959 and Woodwardopteridae Kjellesvig-Waering, 1959.
I have for many years considered that the family Stylonuridae, and in particular the genus Stylonurus, was a catch-all for practically anything that had stylonuroid legs: that is, legs in which the last pair lacked the flat paddle so characteristic of the other eurypterids. In using the ventral shield and the metastoma as the main distinction between families, as Størmer did with respect to the