Sakai, 1939
This new species agrees with Ptychognathus glaber in having a single obscure tooth on either side of the lateral border behind the external orbital angle, but the carapace is perfectly smooth and leaves no vestige of the epigastric ridges. The free margin of the front is medially concave and is lined with depressed fine granules. The anterior half of the lateral borders is sharply cristate, the posterior half feebly convergent and not cristate. Eyestalks are rather slender, the infraorbital border bearing 11 to 12 pearly tubercles which are externally followed by several microscopic ones. The exognath of external maxillipeds is foliaceous and a little broader than ischium, the merus being strongly auriculated at antero-external angle and much broader than long.
The chelipeds are very stout and symmetrical, the arm and wrist are unarmed, the palm much swollen and high, the chela furnished with a tuft of hairs at the base of the outer surface; the fingers are moderately gaping, the immovable finger bears four teeth, of which the two middle ones are very stout and sharpish; the movable finger bears three small teeth near the proximal half. The terminal segment of male abdomen is not so elongate as that of Ptychognathus barbatus. (T. Sakai, 1939: 660)
Type locality: Lan-Yü, Taiwan.
Range: Japan - Amami-shoto and Yaeyama-retto (Sakai, 1976a), Hachijo-jima and Okinawa Prefecture (Muraoka, 1998), Kinokawa, Wakayama (Nomoto et al., 1999), Ginowan, Okinawa (Shokita et al., 2000); Taiwan - Lan-Yü (Sakai, 1939, 1976, Lin, 1949).