108 THE BRITISH REPTILES. 



In the Thecophora the dorsal vertebrae and ribs are expanded 

 into bony plates forming the carapace, and the parietal bones are 

 prolonged downwards to meet the pterygoids directly or indirectly. 

 There are three sub-orders ; in one of these the carapace is very 

 flat, almost round, and covered with leathery skin instead of horny 

 shields, in the other two the carapace (except in one genus) is 

 covered with horny shields. The genus is Caretlochelys, the sub- 

 order to which it belongs being Pleurodira, which consists entirely 

 of freshwater tortoises. These are recognised by the neck bending 

 laterally and being retracted into the space between the forepart of 

 the carapace and plastron, and by the pelvis being united with the 

 shell. They are not represented in this country. 



In the Cryptodira, the third sub-order, the carapace is covered 

 with horny shields. The neck, if retractile, bends in a sigmoid 

 curve in a vertical plane, and the pelvis is not united with the shell. 

 The transverse processes of the cervicals are almost or entirely 

 wanting ; the centrum of the last cervical articulates with the 

 centrum of the first dorsal. The pterygoids are narrow, in the 

 middle and touch along their inner edges. There are six families, 

 only one of which, the Chelonidae, has a British representative. 



In the Chelonidae the neck is incompletely retractile and short ; 

 the temples are roofed over by the expanded parietals and adjoining 

 bones, the parietals being in contact with the squamosals; the shell 

 is heart-shaped, the limbs are paddle-shaped and widened by the 

 enlarged pisiform bone ; in the fore paddle the phalanges of the 

 digits are 2, 3, 3, 2, 2, in the hind-paddle there is one more joint in 

 the fourth digit. This is the family of the Sea-Turtles, and of them 

 we have but one representative, the Hawksbill (Plate xxvii., 73). 



In the Atheca the vertebrae arid ribs are separate from the cara- 

 pace, which consists of small polygonal plates covered with leathery 

 skin. In the skull the parietal bones are without descending 

 processes. The neck is not retractile ; the limbs are in the shape of 

 paddles and without claws. There is but one family, Sphargidae, 

 and this consists of a single genus, which in its turn has but one 

 species, the Leathery Turtle (Plate xxvii., 74). 



Our two Chelonians appear so seldom on our shores that they 

 may be regarded rather as accidental immigrants than as occa- 

 sional visitors. With our seven Saurians it is different. They are 

 all undoubtedly native, all, perhaps, but one which may have ceased 

 to be English, but remains British from its breeding as usual in the 

 Channel Islands. 



In the Saurians the quadrate bone is more or less movable, and 

 has its lower end projecting ; there is no lower temporal arch ; the 

 ribs of the back are single-headed, and join on to a facet at the side 

 of the body of the vertebrae instead of to the junction between the 

 vertebras as in the Chelonians, or to the arches on them as in the 

 Crocodiles and Plesiosaurs ; there is no plastron, nor are there any 

 abdominal ribs, properly so-called ; and the teeth are not implanted 

 in sockets, but fixed direct to the surface of the bones. This, the 

 dominant division of reptiles, is formed of two, or rather three, 

 orders if the Chameleons be separated from the others ; but as these 

 (Rhiptoglossa) are not represented with us, we need say no more 



