The absence, suppression, or failure to develop one member of a series, such as a finger or vertebra.
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
Examples from the Web for excalation
Historical Examples of excalation
In fact, more or less complete “excalation” of the somite takes place.