Some remarks on the subgenus Oligotrochus M. Sars, 1866 sensu Heding, 1935 (genus Myriotrochus, Myriotrochidae, Holothurioidea) with description of two new species

The composition and evolution of the subgenus Oligotrochus [genus Myriotrochus (Myriotrochidae, Apodida, Holothurioidea)] are discussed. In addition to the type species of the subgenus, M. (O.) vitreus (M. Sars, 1866), two other species are transferred to this subgenus: M. (O.) clarki Gage et Billett, 1986 and M. (O.) bathybius H. L. Clark, 1920. Moreover, two new species are described: M. (O.) rotulus n.sp. from the West Galicia coast, Spain, Northeast Atlantic and M. (O.) neocaledonicus n.sp. from the Loyalty Islands Basin, New Caledonia, Pacific. M. (O.) rotulus n.sp. is charactered by wheels with "fused spokes". The spokes in these wheels are swollen and sometimes are fused, leaving small oval holes near the hub. The number of these holes corresponds to the number of fused pairs of spokes and ranges from two up to the total number of spokes. These wheels usually have less hub perforations than spokes, because not all of the spokes are fused. The latter character and a smaller size of the wheels clearly differ M. rotulus from M. bathybius and M. neocaledonicus , which have wheels with hub penetrated by a complete circle of perforations. M. (O.) neocaledonicus n.sp. is characterised by wheels with perforated hub which closely resemble wheels of M. (O.) bathybius . The new species differs from M. (O.) bathybius in having a smaller length of hub perforations and by the shape of these perforations (triangular or ovoid-triangular in M. (O.) neocaledonicus , and ovoid in M. (O.) bathybius ). An identification key for the species belonging to the subgenus Oligotrochus is given.