Hexatrygonidae

...the family Pristidae has a protruding snout with a row of jagged teeth on both sides; the family Rhinobatidae has a long snout but no projections on either side; the family Platyrhinidae has a rounded body plate with fine scales all over its body; the family Rajidae has a few scales on its back and a long, slender tail; the family Dasyatidae has almost no scales and a venomous stinger on its whip-like tail; the family Torpedinidae has no scales at all and has an electric organ between the pectoral fin and the head; the family Myliobatidae has large pectoral fins and a rounded protruding head tip; and the family Mobulidae has bat-ear-like fins (cephalic fins) on the end of its head. Other families found abroad include the freshwater Potamotrygonidae, which are close to the stingray family, the Anacanthobatidae, whose snouts have filament-like protruding tips, and the Hexatrygonidae, which have six pairs of gill slits. Phylogenetically, the ancestor of the guitarfishes differentiated from the sharks around the end of the Jurassic period of the Mesozoic era, about 130 million years ago, and skates, stingrays, and eagle rays appeared in the Cretaceous period, and devil rays differentiated from eagle rays in the Quaternary period of the Cenozoic era and became surface-dwelling.

*Some of the terminology that mentions "Hexatrygonidae" is listed below.

Source | Heibonsha World Encyclopedia 2nd Edition | Information

…吻（ふん）が突き出てその両側にぎざぎざのある歯が並ぶノコギリエイ科Pristidae，吻は長いが，両側に突起のないサカタザメ科Rhinobatidae，体板がまるく，体中に細かいうろこのあるウチワザメ科Platyrhinidae，背中に多少のうろこをもち尾部の細長いガンギエイ科Rajidae，うろこがほとんどなくむち状の尾に毒針のあるアカエイ科Dasyatidae，うろこはまったくなく，胸びれと頭の間に発電器官のあるシビレエイ科Torpedinidae，胸びれが大きく，頭の先端がまるく突き出たトビエイ科Myliobatidae，頭の先にコウモリの耳のようなひれ(頭鰭（とうき）)のあるイトマキエイ科Mobulidaeである。このほかアカエイ科に近い淡水産のPotamotrygonidae科，吻端がフィラメント状に突出するAnacanthobatidae科，鰓孔が6対あるHexatrygonidae科が外国に分布する。　系統的には約1億3000万年前の中生代ジュラ紀末ごろからサカタザメ類の祖先型がサメ類より分化し，白亜紀になってガンギエイ類，アカエイ類，トビエイ類が現れ，新生代第四紀になるとイトマキエイ類がトビエイ類から分化して表層性になったものとされる。…

※「Hexatrygonidae」について言及している用語解説の一部を掲載しています。

出典｜株式会社平凡社世界大百科事典 第２版について | 情報