又稱正形尾.魚類的一種尾鰭，常見於硬骨魚類.正尾的外形完全對稱，形成相似的兩葉鰭片.鰭片由軟鰭條支持，無硬棘.正尾的內部結構不對稱：尾椎骨末端上翹；尾下骨較尾上骨發達.正尾的形態與游速有密切關係，如鰭片深凹，正尾呈新月形，尾柄細而有力則游速快、遊程長，如鮪魚、鮐魚、鯖魚等.如鰭片中間凹陷，尾柄無明顯變化，則游速不快，如鯉魚等.GOOGLE英文翻譯:Also known as being shaped tail. A kind of tail fins of fish commonly found in bony fish. Are completely symmetrical shape of the end to form a similar two leaves Fins. Fin supported by the soft dorsal fin, no hard spine. Are at the end of the internal Structural Asymmetry: tailbone end of the upturned; tail bone under the well-developed compared with the tail bone. are at the end of morphology and swim speed are closely related, such as Deep fins are crescent-shaped tail, caudal peduncle is small and powerful travel speed quick, run-length, such as tuna, mackerel, mackerel and so on. such as the fin the middle of depression, caudal peduncle no significant change, then the travel speed unpleasant, such as carp and so on.
