Crawl
This is Chinese radical number 26, 卩, jié. It can mean to kneel, but in this instance, it is best translated as crawl. The sign occurs immediately before the start of a passageway at the end of the Axial Gallery in Lascaux that was 60cm high and one metre wide.
Reference
Image Credit: Norbert Aujoulat MCC-CNP: Picture 95: Crosier-shaped sign: Lascaux – Movement, Space, and Time: The Red Panel: http://www.american-buddha.com/lascaux.5f.htm