Hidden-necked turtles are distinguished from side-necked turtles by the way they fold their neck into their shell. Hidden-necked turtles retract their neck inwards along the axis of the spine, curving it in an S shape along the spinal plane so that their head moves directly into the shell. Side-necked turtles, in contrast, fold their neck and head sideways and tuck it under the edge of the shell closer to the shoulder so that the head and neck bend at an angle relative to the axis of the spine.
Hidden-necked turtles also differ from side-necked turtles in the structure of their carapace and plastron. The carapace and plastron of hidden-necked turtles are rounder relative to those of the side-necked turtles, whose carapace and plastron are more oval in shape. Additionally, the bones and scutes take on different arrangements in the two clades of turtles.
There are 11 families of hidden-necked turtles some of which include seaturtles, pond turtles, American mud and musk turtles, tortoises and softshell turtles. In total, there are more than 200 species of living hidden-necked turtles.