The turtle shell is a remarkable evolutionary novelty. It is composed of two main parts, the dorsal carapace and the ventral plastron. Between them, on the lateral sides, is a bridge. The carapace contains fifty bones and is supported by the ribs, which grow dorsolaterally rather than ventrally. The critical event in turtle development and evolution is thought to be the "ensnarement" of the ribs by the carapacial ridge, a bulge of ectoderm and mesoderm that resembles the limb bud. The bones of the shell are thought to be formed from the dermis by intramembranous ossification, with contributions from the ribs. We wish to test three hypotheses on the mechanisms governing turtle shell development: (1) that the carapace ridge forms by the co-option of the developmental program that forms the limb buds; (2) that the ribs induce intramembranous ossification in the dermis; and (3) that the dorsal growth of the rib cartilage precursors depends on their encountering the carapacial ridge.