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Why can side-necked turtles not withdraw their heads completely into their shells?

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Illustrations of ten representative chelonians, including spiny softshell turtle, Galapagos giant tortoise, green turtle, alligator snapping turtle, Blanding's turtle, common map turtle, eastern box turtle, eastern painted turtle, spotted turtle, and eastern mud turtle, each labeled with its common and scientific name. A scale bar at the bottom shows size comparison. — turtle Turtles (order Testudines).

Side-necked turtles (suborder Pleurodira) cannot withdraw their heads completely into their shells because of the way their necks are structured to fold. Unlike "hidden-neck" turtles (suborder Cryptodira), which fold their necks vertically to pull the head straight back into the shell, pleurodires fold their necks horizontally to the side.12

Because of this lateral folding mechanism, these turtles cannot retract their heads into the shell for protection. Instead, they exhibit a defensive posture where they lay the head and neck to the side, tucking them beneath the margins of the shell. This distinction in neck movement is a primary criterion used by scientists to differentiate between the two major suborders of turtles.12