for eurypterid species described in the late 19th to early 20th centuries. In 1912, American paleontologists John Mason Clarke & Rudolf Ruedemann declared that the differences between Eusarcus and all related forms of eurypterids were so great that it was "entirely evident" that Eusarcus was distinct from other eurypterids. They referred the Scottish Wenlock-age Eurypterus species E. obesus (described by English geologist Henry Woodward in 1868) to the genus, alongside the Pridoli-Lochkovian-age Czech species E. acrocephalus (described by Austrian paleontologist Max Semper in 1898) on the grounds of both possessing triangular carapaces similar to E. scorpionis as well as an abruptly narrowing postabdomen. Furthermore, Clarke and Ruedemann concluded that Eusarcus was sufficiently similar to the related Carcinosoma to be designated as synonymous with it. As Eusarcus had been named earlier than Carcinosoma, the taxonomical laws of priority dictated that Eusarcus would be the name of the taxon.E. acrocephalus (sometimes referred to erroneously as "E. acrocephala") can be distinguished from the other two species by the strongly recurved lateral sides of its carapace, the oval mesosoma and the first segment of the metasoma being unusually wide and short for the genus. The holotype of E. acrocephalus, UW 1906/V/2 discovered in the Požáry Formation of the Czech Republic, is composed of the carapace and a partial abdomen, portions of which were broken off during bombing runs on Prague during the Second World War, during which the National Museum of Prague was damaged. The obesity of the opisthosomal (abdominal) segments in E. obesus is its most remarkable feature (and is also what lends the species its name), the fourth segment is as wide as the first eight segments combined are long. The surface of the exoskeleton of the carapace and the segments is thin and seemingly lacks ornamentation. In 2014, American paleontologist James Lamsdell suggested that E. obesus may represent a juvenile of the related and contemporary Carcinosoma scorpioides. Unlike other species of Eusarcana, E. obesus does not appear to possess spines on its appendages. Recognition as preoccupied name It was first in 1934, 59 years after its original description, that Eusarcus was recognized as a name preoccupied by a harvestman. The Norwegian geologist Leif Størmer proposed that the name of the taxon should be next oldest available and valid name for the genus, Carcinosoma. During the preparation for his paper on the issue, Størmer also discussed the situation with fellow Norwegian researcher