The earless water rat (Crossomys moncktoni) is a New Guinea rodent, part of the Hydromys group of the subfamily of Old World rats and mice (Murinae). It is the only species of the genus Crossomys. This species is probably most closely related to Baiyankamys. It is still unclear to which species this group is related. It is one of the most aquatically adapted rodents of the world. Names It is known as kuypep in the Kalam language of Papua New Guinea. History of discovery This animal was first described in 1907 by the British mammalogist Oldfield Thomas, based on a single specimen caught by one Mr. C. A. W. Monckton, after whom the species was named, near Brown River, Central Province, south-east Papua New Guinea. Only in July 1950 a second specimen was captured. Since then, several other examples have been caught in the mountains of eastern New Guinea, but the earless water rat remains a rather rare species. The scientific name means "Monckton's fringed mouse", which refers to the collector of the original specimen (C. A. W. Monckton) and to the fringe of hairs on the tail. Relationships The German mammalogist Hans Rümmler placed this rodent (and Parahydromys asper) in Hydromys, but that has not been accepted generally. Lidicker (1968), who studies the morphology of the phallus of New Guinea rodents, speculated that Crossomys might not be as closely related to Hydromys as was then generally thought. Later on, this was supported by the immunological study of Watts & Baverstock (1994). This study placed Crossomys closer to Leptomys, Pseudohydromys and Xeromys than to Hydromys. The American mammalogists Guy Musser and Michael Carleton, in their contribution to the authoritative Mammal Species of the World (3rd ed.), divided the group of murine rodents that had before been called "Hydromyinae" or "Hydromyini" in two "divisions": the Xeromys Division (Leptomys, Pseudohydromys and Xeromys) and the Hydromys Division (Crossomys, Hydromys, Microhydromys, Paraleptomys and Parahydromys [Baiyankamys was added later]). According to them, the morphology of Crossomys is more like the Hydromys Division than the Xeromys Division, and therefore they placed it in the Hydromys group. They supported their opinion with an unpublished study of the Australian biologist Ken Aplin, who also placed Crossomys closer to Hydromys. Helgen (2005) concluded that Crossomys is most closely related to Baiyankamys, which had usually been placed in Hydromys before. Baiyankamys has not been studied genetically. According to his data,