Being in the process of passing out of use or usefulness; becoming obsolete.
Biology Gradually disappearing; imperfectly or only slightly developed. Used of an organ or other part of an animal or plant.
American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.