Monotypic prehistoric chelicerate genera

Megarachne is a genus of eurypterid, an extinct group of aquatic arthropods. Fossils of Megarachne have been discovered in deposits of Late Carboniferous age, from the Gzhelian stage, in the Bajo de Véliz Formation of San Luis, Argentina. The fossils of the single and type species M. servinei have been recovered from deposits that had once been a freshwater environment. The genus name comes from Ancient Greek μέγας (mégas), meaning "great", and ἀράχνη (aráchnē), meaning "spider", and thus, "great spider"; because the fossil was misidentified as a large, prehistoric spider.

Sanctacaris is a Middle Cambrian arthropod from the Burgess Shale of British Columbia. It was most famously regarded as a stem-group chelicerate, a group which includes horseshoe crabs, spiders and scorpions, although subsequent phylogenetic studies have not always supported this conclusion. Its chelicerate affinities regain support in later observations, alongside the reassignment of Habelia optata as a sanctacaridid-related basal chelicerate. It has been placed as a member of the extinct family Sanctacarididae alongside Wisangocaris and Utahcaris.

Pentecopterus is a genus of eurypterid, an extinct group of aquatic arthropods. Fossils have been registered from the Darriwilian age of the Middle Ordovician period, as early as 467.3 million years ago. The genus contains only one species, P. decorahensis, that was previously considered the oldest known eurypterid, surpassing all other Ordovician eurypterids known at the time, such as Brachypterus, in age by almost 9 million years; however, newer discoveries have yielded evidence for eurypterids that were older still (?Carcinosoma aurorae dated back to the Tremadocian). The generic name deriv

Echinognathus is a genus of eurypterid, an extinct group of aquatic arthropods. The type and only species of Echinognathus, E. clevelandi, is known from deposits of Late Ordovician age in the United States. The generic name is derived from the Neo-Latin echino- ("spiny") and the Greek gnáthos ("jaw"), in reference to a spiny endognathary (used to handle food) appendage part of the fossil type material.

Tylopterella is a genus of eurypterid, a group of extinct aquatic arthropods. Only one fossil of the single and type species, T. boylei, has been discovered in deposits of the Late Silurian period (Ludlow epoch) in Elora, Canada. The name of the genus is composed by the Ancient Greek words τύλη (), meaning "knot", and πτερόν (), meaning "wing". The species name boylei honors David Boyle, who discovered the specimen of Tylopterella.

Dibasterium is an extinct genus of euchelicerate, a group of chelicerate arthropods. Fossils of the single and type species, D. durgae, have been discovered in the Coalbrookdale Formation of the Middle Silurian period (Homerian age) in Herefordshire, England (in the United Kingdom). The name of the genus is derived from the Latin words dibamos ("on two legs") and mysterium ("mystery"), meaning "mystery on two legs" and referring to its prosomal (of the head) limbs. The species name durgae comes from Durga, a Hindu goddess with many arms.

Brachyopterus was one of the earliest known eurypterid genera, having been recovered from Middle Ordovician deposits in Montgomeryshire, Wales. Though other species have been assigned to it in the past, Brachyopterus is today recognized as containing one valid species, B. stubblefieldi.

Haliestes is a genus of sea spider (pycnogonid) from the Silurian aged Coalbrookdale Formation of England. It contains a single species, Haliestes dasos. The species was first described by David Siveter et al. in 2004.

Eocarcinosoma is a genus of eurypterid, an extinct group of aquatic arthropods. The type and only species of Eocarcinosoma, E. batrachophthalmus, is known from deposits of Late Ordovician age in the United States. The generic name is derived from the related genus Carcinosoma, and the Greek () meaning 'dawn', referring to the earlier age of the genus compared to other carcinosomatid eurypterids.

Bassipterus ("wing from Bass") is a genus of eurypterid, an extinct group of aquatic arthropods. Bassipterus is classified as part of the family Adelophthalmidae, the only clade within the derived ("advanced") Adelophthalmoidea superfamily of eurypterids. Fossils of the single known species, B. bellistriata, have been discovered in deposits of the Late Silurian age in West Virginia and Maryland, United States. The genus is named after Bass, where most of the fossils have been recovered.

Herefordopterus is a genus of eurypterid, an extinct group of aquatic arthropods. Herefordopterus is classified as the only genus in the family Herefordopteridae,'''' a basal family in the highly derived Pterygotioidea superfamily of eurypterids. Fossils of the single and type species, H. banksii, have been discovered in deposits of Silurian age in Herefordshire and Shropshire, England. The genus is named after Herefordshire, where most of the Herefordopterus fossils have been found. The specific epithet honors Richard Banks, who found several well-preserved specimens, including the first Here

Terropterus is a genus of eurypterid, an extinct group of aquatic arthropods. The type and only species of Terropterus, T. xiushanensis, is known from deposits of Early Silurian age in China.

Offacolus is an extinct genus of euchelicerate, a group of chelicerate arthropods. Its only species, O. kingi, has been found in deposits from the Silurian period (Homerian epoch) in the Wenlock Series Lagerstätte of Herefordshire, England. The genus is named after Offa, a king from the ancient kingdom of Mercia, and colus, a person who dwelled among (this time referring to) the Offa's Dyke. The species name honors Robert Joseph King, a British mineralogist who found the fossils of Offacolus.

Holmipterus is a problematic genus of eurypterid, an extinct group of aquatic arthropods. The type and only species of Holmipterus, H. suecicus, is known from deposits of Middle Silurian age in the Sweden. The generic name honours , a renowned Swedish palaeontologist specialising in arthropods and crustaceans, and the species name suecicus is Latin for 'Swedish'.