3 o8 COMPARATIVE ZOOLOGY OF CHORDATES 



Later, that portion of the head which touches the centrum 

 (capitulum) became distinct from that which abuts against 

 the transverse process (tuberculum), by the reduction of the 

 intervening part of the head. In this way, the typical double- 

 headed or dichocephalous ribs arose. Between the two heads 

 of the rib and the vertebra there is a canal through which 

 the vertebral artery passes ; and this vertebrarterial canal is 

 conspicuous in the cervical vertebrae on to which the cervical 

 ribs are usually fused. 



In many forms, the articular heads of the ribs are degenerate 

 and either the capitulum or the tuberculum may be lost. This 



dr.- 



Fig. 156. — Diagram showing the relations of dorsal (" true ") and ventral 

 ribs, as seen in transverse section. 



c, centrum of vertebra ; co, coelom ; dr, dorsal rib ; e, epaxonic muscles ; 

 g, gut ; h, hypaxonic muscles ; ns, neural spine ; vr, ventral rib. 



secondary single-headed condition must be distinguished from 

 the primitive holocephalous type. 



In the Chelonia, the ribs are expanded into broad flat 

 plates which touch one another and are fused to the dermal 

 bones (osteoscutes) to form the carapace. In Sphenodon, 

 crocodiles and in birds the ribs bear uncinate processes, which 

 extend backwards and overlap the next posterior rib. In 

 many cases, the ribs are in two portions : a dorsal or vertebral, 

 and a ventral or sternal portion. The hindmost ribs do not 

 usually reach the sternum, and they are known as floating 

 ribs. Primitively, all the vertebrae as far back as the middle of 



