Tortoises occur already in the Trias. They reached their greatest development towards the end of the Mesozoic and in the earlier Tertiary periods. They are now comparatively reduced in the number of families and genera, although they are still represented by about 200 species. The sub-class as a whole is cosmopolitan, but does not occur in the colder regions.
Their origin is quite unknown. Of recent groups only the Crocodilia and the Rhynchocephalia come into consideration. Combination of these groups with the Chelonia leads to some unknown forms whence also the Theromorpha have arisen. Palaeontology does not help us, all the leading, main groups of Chelonia having been in existence in the earlier Mesozoic ages, and Palaeozoic Chelonia are still unknown. We can, however, to a certain extent, reconstruct an ideal primordial Chelonian by assigning to it all the ancestral characters actually observed in recent and fossil kinds, and by reducing to simpler conditions those features which we know to be more or less exaggerated specialisations. It is reasonable to assume that originally each metamere, except those of the anterior half of the neck and the posterior half of the tail, carried a transverse series of dermal plates, covered with horny shields, while the trunk, according to the greater bulk of the body, increased in size, converging towards the root of the neck and tail. By concentration, reduction of the number, and increase in the size of some of the remaining plates and shields, the skull assumed its characteristic box-like shape, the neck and tail becoming at the same time free. Chelonia are without doubt descendants of terrestrial, or at least semi-aquatic reptiles, and the marine paddled forms subsequently developed from terrestrial kinds.
Classification of Chelonia.–After many vicissitudes it was recognised that the Chelonia cannot naturally be divided according to the modification of their feet. The Trionychoidea were clearly separated from the rest by Stannius in 1854. Cope, in 1870, was the first to emphasise the important character of the mode in which the neck is either bent sidewards (Pleurodira) or withdrawn in an S-shaped curve in a vertical plane (Cryptodira); and he also separated Sphargis as Athecae from all the other Chelonians, for which Dollo in 1886 proposed the term Thecophora. The division of the latter into recognisable families, based upon reliable, chiefly internal, skeletal, characters, has been effected by Boulenger;[[127]] and his classification has been adopted in the present volume, after intercalation of the more important fossil forms. The relationships between these various families may perhaps be indicated as follows:–
| Chelonia |  | Athecae . . . . | Sphargidae | |||
| Thecophora |  | Pleurodira |  | Pelomedusidae | ||
| Chelydidae–Carettochelydidae | ||||||
| Cryptodira |  | Chelydridae–Dermatemydidae–Cinosternidae | ||||
| Platysternidae | ||||||
| Testudinidae–Chelonidae | ||||||
| Trionychoidea | Trionychidae | |||||
The guiding taxonomic characters are fully mentioned at the head of the different families, and are mostly internal. The following "key," adapted from Boulenger, and based upon external characters, is preferable for practical purposes.
For the position and names of the horny shields see Fig. 61 on p. [315].
Shell covered with horny shields.
Digits distinct, with 5 or 4 claws.
Pectoral shields separated from the marginals by inframarginals.
Tail long and crested. Plastron small and cruciform. North America .......... Chelydridae, p. [338].