XIII 



PHYLUM CHORDATA 



ETH 



ant 



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 lor 

 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 

 transverse processes of the abdominal and sacral vertebrae 

 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 vertebrae 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 trabeculae 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 parietal* 



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



branchiate forms Necturus and Proteus the trabeculse remain, 



even in the adult, as narrow cartilaginous bars, and the chondr< 



cranium is actually of a lower or more embryonic type than 1 



of any other Craniate, with the possible exception 



o -j- f\YV~i O T O 



' In C the Urodela, moreover, the parietals (Fig. 893, P) and frontal* 

 (F) are separate, the parasphenoid (Ps) is not T-shaped, 1 he p 

 tine and pterygoid are sometimes represented by a single 



PR.OT 



EX.OC 



nch 



s ( i Proteus anguinus. The chondro- 



cranium from above, ant. antorbital process ; 

 EX.OC. exoccipital and epiotic ; hi/. mil. 

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

 notochord ; ot. pr. otic process ; po.L P ed ^'^ : 

 PR.OT. prootic ; QU. quadrate ; SP.ETh 

 sphenethmoid. (After W. K. Parker.) 



