What's the difference between heterocercal and homocercal?

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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Homocercal

Definition:

• (a.) Having the tail nearly or quite symmetrical, the vertebral column terminating near its base; -- opposed to heterocercal.

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