TORTOISES AND TURTLES. 41 



with notches on the outer border of those of the front row ; but 

 in another group there are five or more longitudinal rows of these 

 bones, which are generally without marginal notches. In Ophthalmo- 

 saarus (351), of the Kimmeridge Clay, a third bone (the pisiform) 

 articulates with the humerus, an analogous condition obtaining in 

 the hind-limb. Both in Ophthalmosaurus and the allied American 

 Baptanodon the teeth were rudimentary. 



Order YIIL— CHBLONIA. 



Tortoises and Turtles. 

 (Oases 6 to 10.) 



Tortoises, Terrapins, and Turtles, which collectively constitute 

 this order, are distinguished from all other Eeptiles by the toothless 

 horn-covered jaws, and the enclosure of the body in a bony shell, 

 which may or may not be covered with horny shields. The shell, 

 which consists of an upper half, or carapace, and a lower portion, 

 or plastron, is supported by the spines of the vertebrae and the ribs ; 

 and consequently Ohelonians present the unique peculiarity that the 

 shoulder and pelvic girdles are situated within the ribs. The limbs, 

 which are five-toed, may be adapted for walking (Tortoises) or 

 modified into paddles (Turtles). Each rib articulates with the 

 vertebrae by a single head, and the quadrate-bone is firmly united 

 to the skull. This order dates from the Triassic epoch. 



Ohelonians are arranged in two main divisions : the Athecae and the 

 Thecophora. In the former group, now represented by the Leathery 

 Turtle, or Luth, the vertebrae and ribs are free from the carapace 

 (fig. 45), which is composed of small polygonal plates like mosaic, and 

 covered with horny skin. In the second group, the vertebrae and ribs 

 are fused with the carapace (fig. 44), which is composed of a number 

 of bony plates of variable size, the names and relations of which are 

 shown in case 6. In this group, as shown in the figure on page 52, 

 the number and size of the horny shields do not accord with those of 

 the underlying bones. 



The Thecophora are subdivided into the following three groups : 



1. Cryptodira, or those in which the head is retracted in a 



vertical plane by an S-like flexure of the neck (fig. 55), and 

 the pelvis is not attached to the plastron. 



2. Pleurodira, or those in which the head is retracted in a 



horizontal plane by a lateral flexure of the neck (fig. 56), and 

 some of the bones of the pelvis are welded to the plastron. 



