528 UEODELA [CH. 



vertebra are expanded in accordance with the strain put on them 

 by the attachment of the ilium. Of the vertebrae those of the tail 

 are the most primitive since they are composed of all the four pairs 

 of arch-pieces ; but of these only the basidorsals and the basi- 

 ventrals become ossified at first, and joining together form the bulk 

 of the vertebra, each basidorsal, as in fish, becoming connected with 

 the basiventral belonging to the myotome in front, while the dorsal 

 and ventral intercalaries, although likewise fusing together, remain 

 cartilaginous and form the intervertebral cartilage. This either 

 remains continuous and owing to its flexibility acts as a joint, 

 or it becomes More or less separated into two portions which 

 articulate TO&h one another, one forming a cup and the other a ball. 

 Joints in wbi(!h the cup belongs to the posterior end of the vertebrae 

 are called opisthocoelous, e.g., in Desmognathus triton, bu^he most 

 frequent form of joint is one in which the ball forms the posterior 

 and the cup the anterior portion of the vertebra. Such vertebrae 

 are said to be procoelous. The portions 6i the intervertebral 

 cartilage eventually become ossified and joined to the previously 

 formed centrum. When the intervertebral cartilage does not form 

 a joint but remains soft, the bony vertebrae in the dried skeleton 

 appear to have the form of an hour-glass with cups at both sides 

 to be in fact amphicoelous, but they differ fundamentally from the 

 amphicoelous vertebrae of Teleostei in that in these fish a portion 

 of the notochord persists between two adjacent vertebrae, whereas 

 in Urodela the notochord is obliterated between two successive 

 vertebrae. The basiventrals of the tail vertebrae form long 

 haemal arches. 



In the trunk the basiventrals appear only in young larvae ; in 

 the adult they disappear so that the bulk of such vertebrae is formed 

 only by the pair of basidorsals to which the ribs become secondarily 

 attached by the formation of an outgrowth from the basidorsal 

 termed the tubercular process (Fig. 254). 



It is of importance to note that in many of the extinct 

 Stegocephala, e.g., Archegosaurus, the caudal vertebrae were repre- 

 sented by four pairs of distinct arch-pieces, viz. basidorsals, basi- 

 ventrals, and dorsal and ventral intercalaries, while the trunk 

 vertebrae consisted of three separate pairs of pieces, namely the 

 basidorsal, the dorsal intercalary, and the basiventral ; but that in 

 the typical Labyrinthodonts, the highest of the Stegocephala, all 

 these constituent pieces were united into solid vertebrae ; lastly, 

 that in some of the lowest, e.g., in Branchiosaurus, each vertebra 



