8o The Alligator and Its Allies 



mediate portion is present in all but the tenth rib, 

 and wherever present, except in the ninth rib, 

 it articulates distally with the sternal portion. 

 The sternal portions extend medio-cephalad in a 

 direction at right angles to the intermediate portion ; 

 the first two articulate with the sternum, the next 

 six with the xiphisternal horns, and the ninth and 

 tenth are missing. 



TJie Abdominal Ribs (Fig. 26, 4) . While these ribs 

 are membrane bones and are not homologous with 

 the other ribs, they may as well be mentioned at 

 this time. They consist of about seven V-shaped 

 sets of slender bones, the point of each V being 

 directed cephalad. Each V is made up of from 

 two or five slender bones, the nimiber and arrange- 

 ment being subject to considerable variation. 

 The last V of the series (not well shown in the 

 figure) is considerably larger than the rest and is 

 made up of four curved bones that extend around 

 the anterior ends of the pubic bones and are united 

 to them by a broad tough membrane. The first or 

 most anterior V is united by a narrow membrane 

 (not shown in the figure) with the membrane that 

 extends between the xiphisternal horns. All of 

 the V's are more or less connected with each other 

 by fibrous membranes. Since these ribs lie super- 

 ficial to the recti muscles of the ventral body wall 

 they are sometimes missing in carelessly prepared 

 skeletons. 



The Sternum (Fig. 26), The sternum consists of 



