PHYLUM CHORDATA 



279 



perched upon a persistent notochord surrounded by incomplete 

 hoops of bone, twice as numerous as the arches, and alternately 

 dorsal and ventral in position. The former represent centra, the 

 latter inter-centra or ossifications alternating with the centra on 

 the ventral region of the notochord. 



The first or cervical vertebra bears paired articular surfaces for 

 the condyles of the skull, and between them the anterior face of 

 the centrum gives off, in Urodela, a projection called the odontoid 

 process. The Urodela, moreover, have ribs articulating with the 

 transverse processes of the abdominal and sacral vertebrae : they 

 are short bones, forked proxi- 

 mally, and the compressed 

 transverse processes are cor- 

 respondingly divided. The 

 sacral ribs of Urodeles give 

 attachment to the ilia, and 

 the caudal vertebraB bear 

 haemal arches. 



The skull of Urodela differs 

 from that of the Frog in many 

 important respects, the most 

 striking of which is the fact 

 that the trabeculse do not 

 meet either below the brain 

 to form a basis cranii or above 

 it to form a cranial roof. 

 Thus when the membrane 

 bones are removed the 

 cranium (Fig. 892) is com- 

 pleted above and below in 

 the parachordal or occipital 

 region only : anterior to this 

 it has side walls, but no roof 

 or floor, there being above a 

 huge superior cranial fonta- 

 nelle, and below an equally 

 large basi-cranial fontanelle, 

 the former covered, in the entire skull, by the parietals and 

 frontals, the latter by the parasphenoid. In the perenni- 

 branchiate forms Necturus and Proteus the trabecuke remain, 

 even in the adult, as narrow cartilaginous bars, and the chondro- 

 cranium is actually of a lower or more embryonic type than that 

 of any other Craniate, with the possible exception of Cyclo- 

 stomata. 



In the Urodela, moreover, the parietals (Fig. 893, P) and frontals 

 (F) are separate, the parasphenoid (Ps) is not T-shaped, the pala- 

 tine and pterygoid are sometimes represented by a single bone 



PR.OT 



FIG. 802. Proteus anguinus. The ehondro- 

 cranium from above, ant. antorbital process ; 

 EX.OC. exoccipital and epiotic ; Jti/.-nul. 

 hyomaiidibular ; i.n. inter-nasal plate ; nch. 

 notochord ; ot. pr. otic process ; ped. pedicle ; 

 PR.OT. prootic ; QU. quadrate ; SP.ETH- 

 sphenethmoid. (After W. K. Parker.) 



