A new fresh-water crab, Potamon (Geothelphusa) tenuimanus sp. nov. from Okinawa-jima, the Ryukyu Islands

/Diagnosis./ Carapace is of moderate size. Upper surface smooth and flat except frontal region being strongly curved downwards. Legs remarkably slender and long. Merus of the second ambulatory legs almost equal to the length of carapace. It is easily distinguished from the known species of the Ryukyu Islands by slender and long legs. /Description of holotype./ Carapace very flat, but the frontal region abruptly curved downwards. Upper surface almost smooth except the following regions. The upper part of branchial region ornamented with fine granules. The intestinal and lower part of branchial regions furnished with short striae near the posterior margin of carapace. The length of carapace a little shorter than three-fourths the breadth of carapace. Depth of carapace slightly longer than half of the length of carapace. Anterolateral margin forming granulated crest is relatively short and strongly curved at the position of the maximum breadth of carapace. Epigastric lobe distinct and divided by a median groove into two lobes. The cervical groove is formed a deep, short depression between the mesogastric and urogastric regions. Urogastric region is divided into two lobules. Subbranchial region strongly convex. In the abdomen the length of 7th segment slightly longer than half of breadth. Length of 6th segment a little shorter than the maximum breadth. First pleopod very broad at the base being ornamented with pulmose hairs, and is very slender distally. Second pleopod smaller than first pleopod. The shapes of the pleopods are shown in the Fig. 2. Chelipeds unequal, the right one being distinctly larger. In the right chela the upper border of palm slightly shorter than maximum breadth, and a little longer than half of the length of dactylus. Breadth of palm about half of the chela. The movable finger about three-fourths the length of chela, and armed with 16 small teeth on inner margin. The immovable finger armed with a large tooth on one-third from the proximal end, and furnished with 15 small teeth on inner margin. There is a large gap between the both fingers. Ambulatory legs remarkably slender and long, and smooth without spines and hairs except dactylus and propodus being armed with spinules on both upper and lower margins. Second ambulatory leg is the longest in all ambulatory legs, and the merus is equal to the length of carapace. Propodus about five times its breadth, and slightly shorter than dactylus. /Description of paratypes./ Two paratypes are the same features as in the holotype (male) except the cheliped and abdomen. The right cheliped is larger than left one. The larger cheliped is the same shaped as in the left one of the holotype. Dactylus furnished with small teeth on the inner margin, but not armed with a large tooth. In the abdomen the length of 6th segment is about one-third the breadth, and slightly longer than the length of 7th segment being a little longer than the breadth. /Holotype./ Male, Zoological Laboratory, Kyushu University (ZLKU) Cat. No. 9726, Izumi, Motobu-cho, Okinawa-jima, Ryukyu Islands, Aug., 1960, collected by Mr. Seki-Ichi Chinen. /Paratypes./ 2 ovig. females, ZLKU Cat. No. 9367, Katsuu-dake, Motobu-cho, Okinawa-jima, Ryukyu Islands, Aug. 6, 1959, collected by Mr. Shosei Takaezu. These two ovigerous females are dry specimens