and their zygapophyses (processes that interlock adjacent vertebrae) are much more widely spaced--features allowing for a condition called ginglymoidy, and ultimately, their "hidden" neck retraction. Ginglymoidy refers to the double articulation where articulation between the sixth and seventh vertebrae and the seventh and eighth vertebrae allows for bending of the neck into an S shape. Formation of this S shape occurs in one plane that enables retraction into the shell.

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