(Miyake & Chiu, 1965)
Diagnosis: Carapace medium-sized, distinctly larger than Potamon dehaani (White), but not larger than Potamon sakamotoanus Rathbun. Breadth of orbit relatively shorter than the two above-mentioned species. The H-shaped groove of the middle portion of the cervical groove is indicated by a very deep depression. The branchial groove is indicated by a very deep depression. The branchial lobule and the two urogastric lobules can be seen in a deep transverse depression being continued to the H-shaped groove in the middle of the branchial region. Antero-lateral margins form a distinct crest and are not separated to the outer orbital crest. Ambulatory legs more broader than the allied two species. Carapace and legs dark purple in living colour.
Description of types: Carapace a little broader, the breadth being only about one and one-fourth times the length. The upper surface slightly convex fore and aft in anterior regions, and smooth but punctate in all regions. Anterior margins of epigastric and hepatic lobes are distinct by the depression of the frontal and orbital regions, but those of posterior margins indistinct by faint shallow grooves. Epigastric lobe is distinctly divided into two lobes by a median groove which extends backwards. The urogastric region is separated in two lobules laterally by a short distance. The branchial lobule is conspicuous and located to outside of the H-shaped groove in the branchial region.
In the middle of the branchial region there is a broad, deep depression between the branchial lobule and the middle of the postero-lateral region. The upper surface of the branchial region is ornamented with short striae near the postero-lateral margin and a short crest near the postero-lateral corner. This short crest runs outsides and continues to the postero-lateral margin. These remarkable depressions can not be seen in the two allied species.
The antero-lateral margin forms a distinct, continuous crest, and is not beaded in the holotype and allotype, but in one of the paratypes being smaller specimen it is ornamented with same-sized tubercles or granules.
The suborbital, subhepatic and pterygostomial regions are smooth in the holotype and allotype. In the paratypes, however, the suborbital and the subhepatic regions are granulose, and the pterygostomial region is ornamented with short hairs. The pleural groove is distinct and accompanied with a crest on the inner lower margin of the subhepatic region. The crest in the pleural groove is smooth in the holotype and allotype, but it is ornamented with short hairs in the paratypes or smaller specimens. The sternum of the third maxilliped is furnished with short hairs on the posterior margin, but in the two allied species not hairy.
Chelipeds smooth and unequal, right one being larger in the holotype and allotype, but left one larger in the paratypes. Merus with short striae on the upper distal margin. Wrist with a short depression longitudinally and on inner margin armed with a short tooth being accompanied with one more small tooth or tubercle downwards on inner surface. Fingers of the chela armed with small obtuse teeth being counted ten teeth on the movable finger and fifteen ones on the immovable finger in the types. There is a small gap between fingers only in larger or right cheliped in the holotype.
Ambulatory legs smooth except propodus and dactylus being spinose and hairy. Dactylus longer than propodus, and armed with spines on both upper and lower margins. Propodus with spines only on lower margin, and the distal margin being articulated the dactylus is armed with spines. Ambulatory legs are more broader than that of the two allied species. The length of the propodus is about twice as broad as the dactylus.
The sixth segment of the abdomen is longer than the last segment in both sexes and it is distinctly longer than the two allied species. The shape of the first pleopod is the same as that of the two allied species. (Miyake & Chiu, 1965: 595)
Type locality: Alilao village of Shi-men district, Taipei County, Taiwan.
Range: Japan - Ishigaki-jima and Iriomote-jima (Minei, 1973); Taiwan - Taipei County (Miyake & Chiu, 1965, Minei, 1974).