diphycercal

from The American Heritage® Dictionary of the English Language, 5th Edition.

• adjective Having or designating a tail fin in which the vertebral column extends to the tip, with symmetrical upper and lower parts, as in the lungfishes.

from The Century Dictionary.

• In ichthyology, having the tail symmetrical, or consisting of equal upper and lower halves, with respect to the bones which support it, the end of the spinal column or the notochord not being bent upward as is usually the case in fishes. See homocercal, hypural, heterocercal.

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

• adjective (Anat.) Having the tail fin divided into two equal parts by the notochord, or end of the vertebral column; protocercal. See protocercal.

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

• adjective zoology Describing a symmetric tail fin in which the vertebral column extends to the tip