“ In no living thing are the lines of beauty more exquisitely defined than in the crescentic borders of these flukes. At its utmost expansion in the full grown whale, the tail will considerably exceed twenty feet across. ”
 Herman Melville, Moby-Dick (1851). copy citation

add

“The compact round body of its root expands into two broad, firm, flat palms or flukes, gradually shoaling away to less than an inch in thickness. At the crotch or junction, these flukes slightly overlap, then sideways recede from each other like wings, leaving a wide vacancy between. In no living thing are the lines of beauty more exquisitely defined than in the crescentic borders of these flukes. At its utmost expansion in the full grown whale, the tail will considerably exceed twenty feet across. The entire member seems a dense webbed bed of welded sinews; but cut into it, and you find that three distinct strata compose it:—upper, middle, and lower. The fibres in the upper and lower layers, are long and horizontal; those of the middle one, very short, and running crosswise between the outside layers.” source