RHYNCHOCEPHALI 297 



backwards they appear as chevrons, articulating more with 

 the vertebra in front than with the one behind. The bases 

 of the right and left chevrons are frequently fused across. 

 so that the caudal canal is completely surrounded by bone, a 

 feature common in Dinosaurs. Every intercentrum, be it a pair 

 of chevrons, or an unpaired nodule, or cres- 

 cent, extends dorsalwards into a fibro- 

 cartilaginous ring which surrounds the 

 chorda. The centra of the vertebrae are 

 deeply amphicoelous, the cavity being filled 

 throughout life by the chorda ; but the 

 middle of the centra is solid. Most of the F IG . 60. The first three 

 caudal vertebrae are transversely divided cervical vertebrae of 



J Sptenodon. 1, 2, 3, 4, 



into tWO parts, the posterior OI Which Intercentra ; Cj - C 3 , 



carries the greater share of the arches ; centra ; i\y..A 3 , neural 



. arches. 



they resemble in this respect those of lizards, 



.and the lost tail is likewise reproduced. The first three ribs are 

 represented by bands of connective tissue. The first is attached 

 to the side of the first intercentrum ; the second arises from the 

 .second intercentrum, and forms a small tubercle on the side of 

 the second centrum ; the third behaves similarly. The vertebral 

 arteries and lateral strands of the sympathetic nerve-chain pass 

 through these double basal attachments of the reduced ribs. 

 The other ribs are osseous; they possess short capitula which 

 retain their partly intercentral attachment, while the short 

 tubercula are carried by low processes of the centra, not of the 

 neural arches. Already in the thoracic region both capitulum 

 and tuberculum merge into one facet, at first dumb-bell shaped, 

 further towards the tail oval, gradually shifted backwards and 

 dorsalwards upon the middle of the centrum, until the facet 

 reaches and ultimately lies right across the neuro-central suture. 

 The first few caudal vertebrae also possess ribs, which are how- 

 ever very short and fuse with the diapophyses, immediately below 

 which lies the neuro-central suture. 



The whole column consists of twenty -five presacral, two 

 sacral, and about thirty caudal vertebrae. Some of the thoracic 

 ribs have cartilaginous uncinate processes. Three or four pairs 

 of ribs join a typical sternum, into the antero-lateral portion of 

 which are let in the coracoids. The sternum is raised into a 

 low median crest which fuses with the posterior branch of the 



