All observers are agreed that the bones of the plastron and some bones of the carapace are simple membranous bones, arising from centres formed in a pre-existing fibrous membrane. Considerable difference of opinion exists as to the development of the costal and neural plates of the carapace.

Owen speaks of the ossification “as extending from the ribs and neural spines into the substance of the neural and costal plates. The ribs and spines enter into the composition of the carapace.”

Gegenbaur questions whether the ribs of the Chelonia “are not in reality enormously developed transverse processes, and considers that the neural and costal plates have developed in the integument.”

Claus remarks that the spinous processes of eight of “the thoracic vertebræ (2nd to 9th) appear in the middle line as horizontal plates (neural plates), the ribs of the same vertebræ are transformed into broad transverse plates (costal plates).”

Huxley says that the neural plates and the costal plates exist as expansions of the cartilages of the neural spines and ribs of the primitive vertebræ, before ossification takes place. This being the case, the “neural and costal are vertebral and not dermal elements, however similar they may be to the nucleal, pygal, and marginal plates.”