Eusarcana (meaning "true flesh") is a genus of eurypterid, an extinct group of aquatic arthropods. Fossils of Eusarcana have been discovered in deposits ranging in age from the Early Silurian to the Early Devonian. Classified as part of the family Carcinosomatidae, the genus contains three species, E. acrocephalus from Scotland and E. scorpionis and E. spiniferum from the United States.

Eusarcana is known for its odd proportions and features; the broad abdomen, thin and long tail, spined and forward-facing walking appendages and sharp and curved tail spike differentiate it from most other eurypterids, but are shared with other carcinosomatid eurypterids. The triangular carapace, oddly positioned forward-facing eyes differentiate the genus further from its closest relatives. Eusarcana was a moderately large species of eurypterid, measuring 70–80 centimetres (27.6–31.5 in) in length.

Originally described under the name Eusarcus, this name was preoccupied by a genus of living harvestmen in the family Gonyleptidae. Following the discovery of this homonym, the genus was also wrongly recognized as synonymous with the related Carcinosoma and was only given replacement names for the older name decades after the error was discovered, first as Eusarcana in 1942 and later as Paracarcinosoma (assumed to have been named without knowledge of the earlier replacement name) in 1964.

Eusarcana can be differentiated from other eurypterids by the considerably narrow border between the prosoma (head) and opisthosoma (abdomen), which is particularly thin considering the subsequent broad and large ellipse-shape of the abdomen. The postabdomen (or tail) also narrows rather quickly from the preceding segments. Further features distinctive of the genus include that the carapace (segment covering the head) is clearly triangular in shape, with eyes placed on the rim of it and positioned forwards, the fact that all walking legs possess spines and that they decrease in length the further back they were placed as well as the cylindrically shaped and sharp telson (the posteriormost division of the body).

In comparison with many other eurypterids, Eusarcana was a rather large animal, with the largest species (also the type species) E. scorpionis reaching lengths of 80 cm (31.5 in). E. spiniferum was similar in size, reaching lengths of 70 centimeters (27.6 in).

The appearance of Eusarcana is somewhat odd in comparison with other eurypterids, not only in its overall shape and proportions but also in that the surface of its exoskeleton is covered in small scale-like ornamentation that is circular in shape, crowded and small in size which differentiates it from other eurypterids in which such ornamentation is usually triangular. Overall, Eusarcana is mostly similar in appearance to other carcinosomatid eurypterids, particularly Carcinosoma which shares its forwardly positioned eyes, and is primarily defined by the small degree of spinosity on its second to fifth pair of appendages and its curved telson.

Eusarcana was first described as "Eusarcus" by the American geologists August R. Grote and William Henry Pitt based on fossils recovered from the Pridoli-age Buffalo Waterlime of New York State. This name derives from the Ancient Greek εὖ, (eu-) meaning "true", and σάρξ (sarx), meaning "flesh", meaning "true flesh". The designated type species was E. scorpionis. Though Grote and Pitt did not provide a generic diagnosis for the genus, the species was well diagnosed with a number of distinctive characters. Furthermore, the genus of Grote and Pitt was seemingly based solely on outlines and shape, which prompted some researchers, such as the prominent English geologist Henry Woodward, to regard the genus as lacking generic characters and as such being invalid, referring E. scorpionis to Eurypterus on the grounds that several British species of Eurypterus, notably E. scorpiodes and E. punctatus (today recognized as species of Carcinosoma), were similar in shape.

Unbeknownst to Grote and Pitt, Eusarcus had already been named as a genus of extant (currently living) laniatorid harvestmen of the family Gonyleptidae, in 1833 and as such constituted a preoccupied name. The preoccupied nature of the name was not immediately recognized, and it continued to be used for eurypterid species described in the late 19th to early 20th centuries.