300 



ZOOLOGY 



SECT. XII] 



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 investing 

 bones are removed, the cranium (Fig. 947) 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 

 the perennibranchiate forms Necturus and Proteus the trabeculse 



remain, even in the adult, as 

 narrow cartilaginous bars, and 

 the chondrocranium is actu- 

 ally of a lower or more em- 

 bryonic type than that of 

 any other Craniata, with the 

 possible exception of Cyclo- 

 stomata. 



In the Urodela, moreover, 

 the parietals (Fig. 948, P) and 

 frontals (F) are separate, the 

 parasphenoid {Ps) is not' 

 T-shaped, the palatine and 

 vomer are sometimes repre- 

 sented by a single bone {Pt), 

 and the palatine, when dis- 

 tinct, bears teeth. The sus- 

 pensorium is inclined for- 

 wards, 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 hyomandi- 

 bular. There are three or 

 four branchial arches which 

 are large in the perenni- 

 branchiate forms, but undergo more or less reduction in caducibranch ' 

 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. 949) very little of the original car- 

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



EXOC 



nch^ 



Fig. 947.— Proteus anguinus. The ohondro' 

 cranium from above. ant. aiitorlntal process 

 EX.OC. exoccipital and epiotic ; hij.iiul. 

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

 notochord ; ot. 'pr. otic process ; perf. pedicle ; 

 FB.OT. pro-otic ; QU. quadrate ; SF.ETH. 

 sphenethmoid, (After W. K. Parker.) 



