What's the difference between heterocercal and protocercal?

Heterocercal

Definition:

• (a.) Having the vertebral column evidently continued into the upper lobe of the tail, which is usually longer than the lower one, as in sharks.

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Protocercal

Definition:

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

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