SKELETON. 



147 



In the cervical region the ribs are much shorter. They 

 may be freely articulated to 

 the vertebrae (crocodiles, etc.), 

 but usually they are coalesced 

 to transverse processes and 

 centra, the foramen for the 

 passage of the vertebral artery 

 remaining to show the mor- 

 phological relations. Usually 

 caudal ribs are poorly devel- 

 oped, but in some reptiles 

 they may appear on almost 

 every caudal vertebra. 



In some stegocephals, as 

 well as in many reptiles (Hat- 

 teria, crocodiles, ichthyosaurs, 

 pterosaurs, etc.), so-called ab- 

 dominal ribs occur. These 

 are chondrifications or ossifi- 

 cations in the ventral wall of 

 the abdomen, usually behind 

 the true ribs, and external 

 to the rectus muscles. From 

 the fact that these are not 

 homologous with the true ribs, 

 the name gastralia has been 

 given them. They may, as in 

 crocodiles, equal the segments 

 in number ; they are twice as 

 many in Hatteria, while in 

 some stegocephals there are 

 several series of ossicles to 

 the somite. 



Sternum. A sternum or 

 breast bone is absent in all 

 fishes, but occurs in the ma- 

 jority of the higher formes ; _ Y ^' l ^' Pos ! e ' ior v f ; eb ' al "i of 



: . . Testudo grceca. CR, caudal ribs ; /, ilium ; 



but it is as yet an open ques- ^ trunk rib s; SR, sacral ribs. 



