Maja is a genus of majid crabs erected by Jean-Baptiste Lamarck in 1801. It includes the following extant species:
Cancridae is a family of crabs. It comprises six extant genera, and ten exclusively fossil genera, in two subfamilies:
Xanthoidea is a superfamily of crabs, comprising seven families. Formerly, a number of other families were included in Xanthoidea, but many of these have since been removed to other superfamilies. These include Carpilioidea, Eriphioidea, Hexapodoidea, Pilumnoidea and Trapezioidea. Even in this reduced state, Xanthoidea remains one of the most species-rich superfamilies of crabs.
Retroplumidae is a family of heterotrematan crabs, placed in their own (monotypic) superfamily, Retroplumoidea.
The thumbnail crab, Thia scutellata, is a species of decapods, in the family of thiidae, whose carapace resembles a human thumbnail, a dense fringe of long hairs distinctly notched around the edge. Pale pink in colour with red to brown markings. It is found in the North Sea, north-east Atlantic and Mediterranean Sea. It is the only extant species in the genus Thia, although two fossil species are known. Their predators includes the atlantic cod.
Hexapodidae is a family of crabs, the only family in the superfamily Hexapodoidea. It has traditionally been treated as a subfamily of the family Goneplacidae, and was originally described as a subfamily of Pinnotheridae. Its members can be distinguished from all other true crabs by the reduction of the thorax, such that only seven sternites are exposed, and only four pairs of pereiopods are present. Not counting the enlarged pair of claws, this leaves only six walking legs, from which the type genus Hexapus, and therefore the whole family, takes its name. Some anomuran "crabs", such as porcelain crabs and king crabs also have only four visible pairs of legs. With the exception of Stevea williamsi, from Mexico, all the extant members are found either in the Indo-Pacific oceans, or around the coast of Africa.
Metacarcinus is a genus of crabs formerly included in the genus Cancer. It includes nine exclusively fossil species and five extant species, of which four are also known from the fossil record. A molecular study using the cytochrome oxidase I gene does not support the monophyly of this genus.
Cheiragonidae is a small family of crabs, sometimes called helmet crabs, placed in its own superfamily, Cheriagonoidea. It comprises three extant species, Erimacrus isenbeckii, Telmessus acutidens and Telmessus cheiragonus, there are no yet evidences of Cheiragonidae in the fossil record. Many of these crabs were formerly treated as members of the Atelecyclidae.
Actumnus is a genus of crabs in the family Pilumnidae. Alongside the 28 extant species, it has a fossil record extending back into the Miocene.
Matutidae is a family of crabs, sometimes called moon crabs, adapted for swimming or digging. They differ from the swimming crabs of the family Portunidae in that all five pairs of legs are flattened, rather than just the last pair, as in Portunidae. Crabs in the Matutidae are aggressive predators.
Pseudozioidea is a superfamily of crabs, formerly treated in the Eriphioidea, Carpilioidea, Xanthoidea, Pilumnoidea and Goneplacoidea. A number of fossils from the Eocene onwards are known from the family Pseudoziidae. Eleven genera are recognised in three families:
Palicoidea is a superfamily of crabs, comprising the two families Crossotonotidae and Palicidae. Together, they contain 13 genera, including two genera in the Palicidae known only from fossils. The two families were previously treated as two subfamilies in a Palicidae of wider circumscription.
Planes is a genus of crabs in the family Grapsidae that currently comprises three extant species: Planes minutus, Planes marinus Rathbun, 1914, and Planes major (=cyaneus). A further fossil species is known from the Middle Miocene of the Caucasus.
Metopograpsus is a genus of crabs, containing the following extant species:
Panopeus is a genus of crabs, containing these extant species:
Xantho is a genus of crabs in the family Xanthidae, containing five extant species, all restricted to the north-east Atlantic Ocean and Mediterranean Sea, although Xantho granulicarpis is not universally recognised as a separate species from Xantho hydrophilus:
Paraxanthias is a genus of crabs in the family Xanthidae, containing one exclusively fossil species and the following extant species:
Xanthias is a genus of crabs in the family Xanthidae, containing two exclusively fossil species and the following extant species:
Zosimus is a genus of crabs in the family Xanthidae, containing the following species:
Dynomenidae is a family of crabs in the superfamily Dromioidea mostly found in Madagascar. There are nineteen genera in this family: five extant and fourteen known from fossils:
