xin 



PHYLUM CHORDATA 



335 



usually replaced by inferior arches ; but Sphenodon, the Chelonia, 

 and Crocodilia have caudal ribs which become fused with the 

 vertebrae. In the Lacertilia only a small number (three or four) of 

 the most anterior of the thoracic ribs are connected with the 

 sternum by cartilaginous sternal ribs ; the rest are free, or are 

 connected together into continuous hoops across the middle line. 

 In the so-called Flying Lizards (Draco) a number of the ribs are 

 greatly produced, and support a pair of wide flaps of skin at the 

 sides of the body, acting as wings, or rather as parachutes. In 

 Sphenodon (Fig. 995) and Crocodilia (Fig. 994) each rib has con- 

 nected with it posteriorly a flattened curved cartilage, the uncinate. 



In the Chelonia (Fig. 997) the total number of vertebrae is always 

 smaller than in the members of the other orders. The cervical 

 ribs are small and fused with the vertebrae. The cervical and the 

 caudal are the only 

 regions in which the 

 vertebrae are movable 

 upon one another. The 

 vertebrae of the trunk, 

 usually ten in number, 

 are immovably united 

 with one another by 

 means of fibre-cartila- 

 ginous intervertebral 

 discs. Each of the 

 neural spines, from the 

 second to the ninth in- 

 clusively, is flattened 

 and fused with a flat 

 plate of dermal origin, 

 the neural plate (Fig. 

 998), and the row of plates thus formed constitutes the median 

 portion of the carapace. The ribs are likewise immovable ; a short 

 distance from its origin each passes into a large bony dermal costal 

 plate, and the series of costal plates uniting by their edges form a 

 large part of the carapace on either side of the row of neural plates. 

 The carapace is made up of the neural and costal plates supple- 

 mented by a row of marginal plates (Figs. 997 and 998) running 

 along the edge, and nuchal and pygal plates situated respectively in 

 front of and behind the row of neural plates. In some cases the 

 neural plates (Chelodina) and even the costal plates and ribs 

 (Testudo loveridgii) are absent. 



The bony elements of the plastron of the Chelonia are an anterior 

 and median plate (entoplastron) and four pairs of plates which 

 are termed in their order from before backwards epiplastra, hyo- 

 plastra, hypoplastra, and xiphiplastra. The median element 

 probably corresponds to the interclavicle or episternum of other 



Po 



FIG. 996. Anterior vertebrae of young Crocodile. 

 A. atlas ; Ep. axis ; h. articulation of atlas wita 

 axis ; IS. intervertebral discs ; o. pro-atlas ; Ob. 

 neural arches ; Po. odontoid bone ; Ps. neural spines ; 

 Pt. transverse processes ; R, R, 1 R, 2 ribs ; s. arch 

 of atlas ; u. median piece of atlas ; WK. centra. 

 (From Wiedersheim's Comparative Anatomy.) 



