hypural

from The Century Dictionary.

• Situated beneath or on the under side of the tail: specifically applied in ichthyology to bones beneath the axis of the tail, supporting finrays.
• noun In icnth., the bone winch supports the caudal fin.

from the GNU version of the Collaborative International Dictionary of English.

• adjective (Anat.) Under the tail; -- applied to the bones which support the caudal fin rays in most fishes.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• adjective anatomy Under the tail; applied to the bones that support the caudal fin rays in most fishes.