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Sakai, 1955

The carapace is perfectly flat and glabrous but the surface near the frontal and lateral borders is obscurely punctate. The frontal margin is sinuate, not much produced beyond the level of the orbits; the two epigastric ridges are obscurely demarcated just behind the frontal border.
The antero-lateral borders are somewhat crested, with a vestige of incision behind the external orbital tooth. The postero-lateral borders are convergent posteriorly; the surface near these borders is depressed but not clearly demarcated. The infraorbital ridge bears ten to eleven tubercles, which are depressed and obscure; in Ptychognathus ishii, which is the nearest kin of this species, it is furnished with ten to twelve pearl-shaped tubercles.
The external maxillipeds are almost identical with those of Ptychognathus ishii in general aspects, the exognath being foliaceous in shape, distinctly broader than ischium, and the merus extremely auriculated at antero-external angle, much broader than long.
Chelipeds are stout and symmetrical, the arm and wrist are unarmed, the inner angle of the latter bluntly cristate but never forming a tooth. The chela is furnished with a tuft of hair on the outer proximal surface, the hair does not extend onto the inferior surface of the palm. The fingers are not much gaping, the immovable finger bears four blunt teeth, of which the proximal two are very small, the movable finger bears about nine small, blunt teeth, of which the distal three or four are indistinct. The tip of both fingers is slightly hooked. (Sakai, 1976a)

Type locality: Hachijo-jima, Japan.
Range: Japan - Hachijo-jima (Sakai, 1955b, 1976); Taiwan.

Ptychognathus hachijyoensis