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(MacGilchrist, 1905)

The carapace is broadly rhomboidal and its upper surface smooth and the regions fairly well defined. The gastric region alone is minutely granulated. The front is well produced and obtusely bidentate, with a median groove on the upper surface. The orbital fissures are all distinct. The anterior boundaries of the pterygostomial regions are divided into three lobes, the median one of which is most prominent and is completely fused with the infraorbital lobe. The hepatic regions are slightly swollen, there is a tubercle in the middle point of the subhepatic regions.
The lateral angles of the carapace are produced into a strong process at the junction of the antero-lateral and postero-lateral borders. This process is exceedingly variable in individuals, but it seems, not on account of sex or age. In the typical species, this process is very long (being about half the length of the carapace), and quite straight, tapering from the base and is acuminate at the tip. There are three males of such typical specimens before me, one from the Seto M.B.L. and the others from Tosa Bay, the former has the processes shorter than half the length of the carapace, directing somewhat forward; the latter has the processes as long as those of the typical species but are asymmetrical, the process of one side being very obtuse. There are also 1 female and 1 male, from Seto M.B.L., 3 females, from Tosa Bay; these specimens have the lateral processes varying in length but equally rounded and truncated at the tip, not tapering at all from the base. In every case, these processes are covered with granules at the base.
The posterior border of the carapace is armed with a pair of papilliform tubercles, which are various in shape, probably correlating with the length and shape of the lateral processes. In the typical specimen, they are constricted at the base, where it is granulated, and the tips are obtusely pointed. In the specimen, which has the lateral processes truncated or rounded, these tubercles are also short and rounded. The intestinal region is convex, armed with a low tubercle in the middle point, but this tubercle is sometimes rudimentary.
The merus of the external maxillipeds is twisted, as the outer border is bevelled and depressed, forming an inner septum of the efferent branchial channel; the exognath is very small and falls short of the tips of the merus, thus spacious openings of efferent branchial channels are left uncovered.
Chelipeds are very slender and cylindrical, the arm is curved forward and has no distinct granules; the palm is as broad as the wrist at the base but tapers very strongly distally, so that the distal half appears almost filiform. The fingers are half as long as the palm in juvenile specimen, but two thirds as long as the palm in the full-grown specimen. The dactyli of the ambulatory legs are furnished with hairs on both sides.
Abdomen of male is very elongated and consists of five pieces; that of the female also consists of five pieces, having a low median tubercle near the base of the fourth segment. (Sakai, 1937)

Type locality: Persian Gulf, "Investigator" stn 292, 96 m.
Range: Persian Gulf (MacGilchrist, 1905); Japan - Kii Peninsula and Tosa Bay (Sakai, 1937a), Sagami Bay (Sakai, 1965b), Sagami Bay, Kii Nagashima, Kii Minabe and Tosa Bay (Sakai, 1976a), Tosa Bay (Miyake, 1983), Nanki Shirahama, Kii Minabe, Seto, Ehime Pref., and Tosa Bay (Muraoka, 1998); East China Sea (Takeda & Miyake, 1970a); Taiwan - Tingch'ieting (Lin, 1949), off Chilung (Takeda & Miyake, 1970a); China - Guangdong including Hainan Island (Dai & Yang, 1991); Vietnam; South China Sea (Serène & Lohavanijaya, 1973); Philippines - south of Manila Bay (Serène & Vadon, 1981), south of Manila Bay and Mompog Pass (Chen H., 1989); Australia - Cape Moreton (Campbell, 1971); 28-204 m.

Arcania cornuta