背鰭10〜11棘，11〜13軟条，臀鰭3棘，8〜10軟条，胸鰭17〜19軟条，腹鰭1棘，5軟条，鰓耙数1+7〜8，有孔側線鱗数87〜98。体長に対する体高の割合は35.7〜40.3％，頭長は38.9〜40.8％，眼径は5.6〜6.7％，両眼間隔は10.3〜11.8％，吻長は12.3〜12.4％，上顎長は19.1〜20.5％，尾柄高は11.6〜13.4％，胸鰭長は18.0〜19.9％，腹鰭長は19.9〜23.5％。体は高く長楕円形で，側扁し，体長は体高の2.5〜2.8倍。頭部は大きく，体長は頭長の2.4〜2.6倍。頭部背面の眼の上後方に1骨質隆起があり，また頭の上部外郭は眼の上後方でわずかにくぼむ。前鰓蓋骨の隅角部には鋸歯状の棘がある。主鰓蓋骨には1骨質隆起があり，その後端に1棘をそなえる。鱗は非常に小さなふし鱗で，はがれにくい。頭部，鰓蓋部，主上顎骨も被鱗するが，眼の上縁の骨質部は露出する。眼は小さく，頭長は眼径の6.1〜7.0倍。口は大きく，上顎の後端は眼の後縁下に達する。下顎は上顎より著しく前方に突出する。両顎歯は小さな円錐歯で，歯帯を形成する。背鰭，臀鰭，腹鰭，の棘は非常に強い。胸鰭は短く扇状。尾鰭の後縁は截型かやや湾入し，尾柄は高い。側線は体側中央部をやや波打ちながら走る。体の背面は青黒色で，腹面は白い。体色は成長による変異が大きく，小型魚では腹面まで青黒色で，尾鰭に小さな黒斑が散在し，尾鰭後縁は白く縁どられる。背鰭，臀鰭および胸鰭の角鰭膜は黒味をおびる。 　ウルガイからアルゼンチン北部の水深40〜80m付近に分布する。 　最大体長1.5m。体重45kg。

（稲田伊史）

　3 from Argentina (356-448 mm SL), FSFL EL 989, EM 19, 318. 　D �]-�]�T, 11-13; A �V, 8-10; P₁ 17-19; P₂ �T, 5; GR 1+7-8; LLS 87-98.
HL 38.9-40.8% of SL; ED 5.6-6.7; BD 35.7-40.3; SN 12.3-12.4; IO 10.3-11.8; UJ 19.1-20.5; CP 11.6-13.4; P₁L 18.0-19.9; P₂L 19.9-23.5.
Body deep, oval in cross section and compressed posteriorly; body depth 2.5-2.8 times in SL. Head large, its length 2.4-2.6 times in SL. A bony ridge on upper part of eye heightens dorsal profile. Posterior margin of preopercle finely serrated. Opercle has upper margin with a small weak spine, posterior margin with a bony ridge and a strong spine. Scales very small, ctenoid, covering head, gill-covers and maxillae, but lacking from bony ridge of upper eye margin. Eye small, its length 6.1-7.0 times in head length. Mouth large, end of upper jaw extending beyond posterior margin of eye. Nostrils situated close together at anterior part of eye. Lower jaw protruding much further anteriorly than upper. Teeth on both jaws small, conical, forming tooth bands. Spines of dorsal, anal and pelvic fins very strong. Pectoral fin short and fan-shaped. Posterior margin of caudal fin usually truncated but slightly concave in large individuals; caudal peduncle deep. Lateral line wavy, and running along the mid part of body. Gill-rakers stumpy. Back of body dark blue, belly whitish. Color of body varies with size： sometimes bluish black below in young fish, small black spots scattered on caudal fin, and whitish on caudal margin. Dorsal, anal and pectoral fin membranes blackish. 　The Atlantic Ocean. In the eastern Atlantic, it is known from the coast of Norway to southern Angola (Smith, 1981). In the western Atlantic, it occurs on the continental shelf and slope from Uruguay to northern Argentina (Figueiredo and Menezes, 1980). 　Maximum length 1.5 m SL, usually 80 cm SL. Taken on handlines and in bottom trawls (Smith, 1981).

(Tadashi INADA)

Distribution of Polyprion americanus in Patagonia.

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