CHAP. IX CHELONIA CLASSIFICATION 313 



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 (Pleurodira) or withdrawn in an S-shaped curve in a vertical 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 recog- 

 nisable families, based upon reliable, chiefly internal, skeletal, 

 characters, has been effected by Boulenger ; ^ 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 



Chelonia- 



Thecophora- 



, ,. ^Pelomedusidae 



\ Chelydidae — Carettoclielydidae 

 Chelydridae — Dermatemydidae- 



Cinosternidae 

 Platysternidae 

 Testudinidae — Clielonidae 

 Trionychoidea Trionycliidae 



Cryptodira 



^ Cat. Chelonians, Brit. Mtis. 1889. 



