xm 



PHYLUM CHORDATA 



291 



ctril 



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 investing 

 bones are removed, the cranium (Fig. 958) is completed 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 fontanelle, 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 J the perenni- 

 branchiate forms Necturus and 

 Proteus the trabeculse remain, 

 even in the adult, as narrow 

 cartilaginous bars, and the chon 

 drocranium is actually of a lower 

 or more embryonic type than 

 that of any other Craniata, with 

 the possible exception of Cyclo- 

 stomata. 



In the Urodela, moreover, the 

 parietals (Fig. 959, P) and 

 frontals (F) are separate, the 

 parasphenoid (Ps) is not T-- 

 shaped, the palatine and vomer 

 are sometimes represented by 

 a single bone, the vomero- 

 palatine (Vop), bearing teeth. 

 The suspensorium is inclined 

 forwards, as in the tadpole, not 

 backwards, as in the adult Frog. 

 The hyoid arch is large, and its 

 dorsal end may be separated as 

 a hyomandibular. There are 

 three or four branchial arches 

 which are large in the perenni- 

 branchiate forms, but undergo more or less reduction in caduci- 

 branch species, never, however, forming such a simple structure as 

 that seen in the Frog. The stapes has no columella attached to 

 it, and, in correspondence with this, there is no tympanic cavity or 

 membrane. 



In the Anura there is a very wide range of variation in the skull. 

 Among the most important points are the presence, in a few species, 

 of small supra- and basi-occipitals, and the fact that in others 

 the roofing investing bones are curiously sculptured and so strongly 

 developed as to give the skull a singularly robust appearance. 



In the Gymnophiona (Fig. 960) very little of the original car- 

 tilage remains in the adult state, but the investing bones are 



PR.OT 



EX.OC 



nch. 



FlQ. 958. Proteus anguinus. The chondro- 

 cranium from above, ant. antorbital process ; 

 EX.OC. exoccipital and epiotic ; hy.md. 

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

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

 PR.OT. pro-otic ; QU. quadrate; SP.ETH. 

 sphenethmoid. (After W. K. Parker.) 



