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the first to emphasise the important character of the mode in which the neck is either bent sidewards (PLEURODIRA) or withdrawn in an S-shaped curve in a vertical plane (CRYPTODIRA); and he also separated _Sphargis_ as ATHECAE from all the other Chelonians, for which Dollo in 1886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:– { ATHECAE Sphargidae { { { _Pleurodira_ { Pelomedusidae { { { Chelydidae–Carettochelydidae { { CHELONIA { THECOPHORA { _Cryptodira_ { Chelydridae–Dermatemydidae– { { { Cinosternidae { { { Platysternidae { { { Testudinidae–Chelonidae { { { { _Trionychoidea_ Trionychidae {314}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.