VERTEBRA AND RIBS 



37 



cranial elements of the next somite are a pair of cranineurals 

 (pleurocentra) and a corresponding pair of cranihaemals (hypocentra 

 pleuralia), the whole forming a rhachitomous vertebra (p. t,2)- 



In other Stegocephals there was apparently a fusion of the caudal 

 parts with each other, and a similar union of the cranials, resulting 

 in an embolomerous condition (fig. 42, D) with centra and inter- 



FiG. 41. — Trunk vertebrae of Cacops (Williston, 'lo). ha, hcemals; np, neurapophysis; 



pi, pleurapophysis. 



centra as in Amia. Other modifications give other shapes. In the 

 highest Stegocephals, as shown in the larvae of Mastodonsaurus, 

 centra and intercentra have fused, forming true amphicoelous verte- 

 brae, the notochord continuing through the dicentral canal. Other 



Fig. 42. — Stegocephalian and genoid vertebra; (Zittel, Woodward). A, phyllo- 

 spondylus; B, rachitomous (Ckelydosaurus); C, Callopterus; D, embolomerus 

 {Eurycormus). hs, hypocentrum arcale; hp, hypocentrum pleurale; w/), neurapophysis; 

 ns, neural spine. 



genera apparently have lost the cranial parts, the vertebrae being 

 formed entirely by the caudal half sclerotome. 



Two more types of centra occur in Stegocephals. The lepo-' 

 spondylous type is a ring of skeletal material restricting the notochord 

 intervertebrally, allowing it to expand between the centra. Increase 

 in length of the ring, as in Urodeles, results in amphicoely. In 



