Eurypterus ( ) is an extinct genus of eurypterid, a group of organisms commonly called "sea scorpions". The genus lived during the Silurian period, from around 432 to 418 million years ago. Eurypterus is by far the most well-studied and well-known eurypterid. Eurypterus fossil specimens probably represent more than 95% of all known eurypterid specimens. There are fifteen species belonging to the genus Eurypterus, the most common of which is E. remipes, the first eurypterid fossil discovered and the state fossil of New York. Members of Eurypterus averaged at about in length, but the largest individual discovered was estimated to be long. They all possessed spine-bearing appendages and a large paddle they used for swimming. They were generalist species, equally likely to engage in predation or scavenging. Discovery The first fossil of Eurypterus was found in 1818 by S. L. Mitchill, a fossil collector. It was recovered from the Bertie Formation of New York (near Westmoreland, Oneida County). Mitchill interpreted the appendages on the carapace as barbels arising from the mouth. He consequently identified the fossil as a catfish of the genus Silurus. It was only after seven years, in 1825, that the American zoologist James Ellsworth De Kay identified the fossil correctly as an arthropod. He named it Eurypterus remipes and established the genus Eurypterus in the process. The name means 'wide wing' or 'broad paddle', referring to the swimming legs, from Greek ( 'wide') and ( 'wing'). However, De Kay thought Eurypterus was a branchiopod (a group of crustaceans which include fairy shrimps and water fleas). Soon after, Eurypterus lacustris was also discovered in New York in 1835 by the paleontologist Richard Harlan. Another species was discovered in Estonia in 1858 by Jan Nieszkowski. He considered it to be of the same species as the first discovery (E. remipes); it is now known as E. tetragonophthalmus. These specimens from Estonia are often of extraordinary quality, retaining the actual cuticle of their exoskeletons. In 1898, the Swedish paleontologist Gerhard Holm separated these fossils from the bedrock with acids. Holm was then able to examine the almost perfectly preserved fragments under a microscope. His remarkable study led to the modern breakthrough on eurypterid morphology. More fossils were recovered in great abundance in New York in the 19th century, and elsewhere in eastern Eurasia and North America. Today, Eurypterus remains one of the most commonly found and best known eurypterid genera,