286 THE VERTEBRATE SKELETON. 



head, and the position of the facet is subject to a consider- 

 able amount of variation, thus in the Squamata it lies on the 

 centrum, and in the Sauropterygia on the neural arch, while 

 in the Chelonia the rib articulates with the contiguous parts 

 of two centra instead of directly with one. 



In most reptiles a greater or smaller number of ribs are 

 united ventrally with a sternum ; but in snakes a continuous 

 series of similar ribs, all articulating freely with the vertebral 

 column, extends from the third cervical vertebra to the end of 

 the trunk. The number of ribs connected with the sternum 

 varies from three or four in Lizards to eight or nine in Croco- 

 diles. Those which reach the sternum are nearly always 

 divided into vertebral, sternal, and intermediate portions, and 

 as a rule only the vertebral portion is completely ossified. In 

 Crocodiles a number of sternal ribs are connected with a 

 cartilaginous arch, which is attached to the hind end of the 

 sternum, and represents the xiphisternum. The sacral ribs 

 connecting the vertebral column with the ilia are very 

 distinct in Crocodiles; in these animals and Sphenodon the 

 vertebral ribs have backwardly-projecting uncinate processes 

 as in birds. 



In the curious arboreal lizard, Draco volans, the posterior 

 ribs are long and straight, and support a parachute-like expan- 

 sion of the integument used in its long flight-like leaps. In 

 Chelonia the ribs are generally combined with the carapace. 



In Ichthyosauria, Sauropterygia, Crocodilia and Sphenodon, 

 abdominal splint ribs occur ; and probably all except the first 

 of the paired ossifications forming the plastron of Chelonia are 

 of similar character. Abdominal ribs have quite a different 

 origin from true ribs, for while true ribs are cartilage bones, 

 abdominal ribs have no cartilaginous precursors, but are 

 simply the ossified tendons of the rectus abdominalis muscle. 



