410 Vertebrata. 



plate. The lower portion of this ring receives the odontoid process, 

 and is separated from the upper, through which the spinal cord 

 passes, by a broad connective tissue ligament. The thoracic ribs 

 consist of an upper bony region and a lower portion, which is often 

 cartilaginous; the latter is sometimes, e.g., in Crocodiles, divided into 

 two parts : from the bony portion a flat backwardly-directed 

 process {processus uncinatus) is occasional! j- given off (Crocodiles). 

 The anterior, or true, ribs are attached to the sternum, in Lacertilia, 

 Crocodilia, and, many extinct forms ; the posterior ribs are free (false 

 ribs) ; no such distinction can be made in the Chelonia and Ophidia,, 

 for a sternum is not present. The ribs are partly fused to the dermal 

 skeleton in Chelonia. In Crocodilia, small ribs, which for the most 

 part articulate by two heads, like the thoracic ribs, occur on all 

 the cervical vertebrse ; the same points may be observed in Lizards,, 

 where, however, there are none on the atlas. The posterior cervical 

 ribs become successively longer, so that there is a gradual transition 

 from cervical to thoracic vertebra;.* Azygos, forked bones, the 

 haemal arches, occur ventrally, in Lizards and Crocodiles, 

 between the caudal vertebrae, but have not coalesced with them. The- 

 sternum (Fig. 339) which is absent from the Chelonia and 

 Ophidia, is usually a short rhomboidal plate, which sometimes [e.g.,. 

 in Crocodiles) is prolonged posteriorly into a long narrow process; 

 it is cartilaginous, and is usually calcified. Connected with the 

 sternum anteriorly is a flat, longish membrane bone, the epi- 

 sternum, which partly covers it and is often drawn out anteriorly 

 into two processes one on either side. 



The skull consists principally of bone, and in many Reptiles is- 

 compressed between the orbits to form a perpendicular plate 

 of cartilage or even partly of mere fibrous connective tissue, the- 

 interorbital septum; the brain is situated behind this plate; in front of 

 it lies the olfactory organ. There is only one occipital condyle 

 below the foramen magnum. The premaxillaet and m a x i 1 1 £e 

 are firmly attached to the skull, as are also the bones formed in the- 

 place of the palato- quadrate ; viz., most posteriorly, the quadrate 

 which bears the articular facets for the lower jaw ; in front of this the 

 pterygoid; and still further forward the palatine ; the two> 

 last bones extend forwards from the quadrate, inwards from the large- 

 maxillae. The extraordinary mobility possessed by the palato-pterygo- 

 quadrate arcade, in connection with the maxilla, in Snakes, is 

 remarkable ; the quadrate in Lizards is also, to some extent, movable, 

 but quite immovable in the Crocodilia and Chelonia, in which groups, 

 the palatines, pterygoids and maxillte are fixed. The lower jaw 



* There is a number of narrow membrane bones, the so-ealled abdominal 

 ribs, in the abdominal wall of Crocodiles, which must not be confused with true ribs. 

 They have no connection with vertebras, and are, not like ribs, preformed in cartilage. 



t In Snakes and many Lizards the premaxillse are fused. 



