Contrary to Webster 1913, ichthyologists distinguish between diphycercal and protocercal caudal fins. Protocercal is reserved for species with notochords, not a true vertebral column. diphycercal looks similar but is reserved for bony fishes (Osteichthyes). Similar in appearance (but again, different in structure) to isocercal.

Pro`to*cer"cal (?), a. [Proto- + Gr. the tail.] Zool.

Having a caudal fin extending around the end of the vertebral column, like that which is first formed in the embryo of fishes; diphycercal.

 

© Webster 1913.

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