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 They supported their opinion with an unpublished study of the Australian biologist Ken Aplin, who also placed Crossomys closer to Hydromys.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, the Crossomys-Baiyankamys group is most closely related to Hydromys and Parahydromys, though he did not give material to support his opinion.
The back is greyish brown, and the belly is white. The fur is soft and water-proof. The top half of the tail is light grey, and the bottom half is white. The forefeet and claws are very small, but the hindfeet are very large. The upper lip contains a row of short, strong brushes, which may be used for rasping. The external ear (the pinna (anatomy)) is reduced to a small oval that does not or hardly protract above the fur. It is possible that the ear canal can be closed. The brain is rather large, like in many aquatic animals. The nasals are relatively small, just like the palate and the molars. The bullae are small. The rostrum is narrow. The Earless Water Rat is a medium-sized rat, about as large as its close relative Baiyankamys. The head-body length is 175 to 200 mm (6.9 - 7.9 inch) (based on four specimens), tail lengh is 212 to 260 mm (8.3 - 10.2 inch) (4), hind foot length is 44 to 53 mm (1.7 - 2.1 inch) (4), ear length is 1 to 4,5 mm (0.2 - 7.9 inch) (2), and weight 165 g (5.8 ounce avoirdupois) (1). Females have 0+2=4 mammae (no thoracic and two inguinal pairs), the same number as most other Australasian rodents.
The Earless Water Rat and Baiyankamys are related because they share the following characters: tail much longer than head-body length; soft, thick, greyish dorsal coat; long, narrow rostrum with a narrow top; very narrow canines; very narrow mesopterygoid fossae; narrow zygomatic arches with a high squamosal root. B. habbema also has the reduced external ears of the Earless Water Rat.
At night it sleeps in holes at the river bank, but at day it is active and hunting for tadpoles, worms and river insects (mostly larvae). The animal gets only one young at the same time. It is only captured by Telefol hunters when river levels are low.
It is called kwypep by the Kalam tribe (Madang Province), possibly ogoyam by the Telefol (Sandaun Province) and momo by the Rofaifo (Southern Highlands Province), although that name is also used for other water rats, like the common Rakali. Some local names can be translated as "water sugar glider", which refers to the similarity of the fur of these two species.
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