SKULL 97 



The hyoid arch consists on either side of a large ceratohyal, 

 and in Ceratodus a small hyomandibular and hypohyal, as well as 

 a median basihyal, are also present. The five branchial arches 1 

 are comparatively small and weak, and some, or even all of them, 

 may be entirely unsegmented (Lepidosiren) . 



The Dipnoi constitute a very ancient group, which must have 

 diverged from the main piscine stem at a very early period, for 

 they occur in the Trias and Carboniferous, and even extend into 

 the Devonian and possibly into the Silurian. 



Amphibians. 



Urodela. The skull of tailed Amphibians is distinguished 

 from that of bony Fishes in general principally by negative 

 characters, on the one hand by the presence of less cartilage in the 

 adult, and on the other by a reduction in the number of bones 

 (Fig. 72). In brief, its structure is in many respects simpler, and 

 becomes modified in adaptation to the different mode of life. 

 Moreover, no nerve-apertures are present in the occipital region 

 behind that for the vagus ; but as this region extends to a slight 

 extent posteriorly to the vagus foramen, it appears that a reduction 

 has here taken place. The occipital part of the skull has the form 

 of a neural arch, united with the auditory capsules above and 

 broadening out below where it abuts against the notochord, form- 

 ing a basal plate primarily including vertebral elements, on the 

 posterior surface of which are two occipital condyles, as in all other 

 Amphibians (Figs. 60 and 72). An exoccipital bone is developed 

 on either side. 



The platybasic cranium is not laterally compressed in the 

 orbital region, and the brain, flanked by the cartilaginous and 

 bony cranial walls, extends between the orbits as far as the olfac- 

 tory capsules, at which point the cranial cavity is closed by a mem- 

 branous (Triton) or cartilaginous (Salamandra) ethmoid region 

 (lamina cribrosa), perforated by the olfactory nerves, or in certain 

 cases by special modifications of the frontal bones (Proteus, Sala- 

 mandra perspicillata). The anterior part of the lateral cranial 

 walls may be ossified as an orbitosphenoid. The well-developed 

 auditory capsules are connected with one another dorsally by a 

 narrow cartilaginous bar (tectum synoticum) all that remains 

 of such an extensive cartilaginous roof as is seen in Elasmo- 

 branchs : this is retained in all the higher Vertebrates. In the 

 ossification of the capsules the prootics take the chief part, and 



1 In Protopterus, a delicate cartilaginous rod arises from the first branchial 

 arch (Fig. 71), concerning the homology of which opinions differ. It may 

 represent the first branchial arch (and in this case the number of branchial 

 arches is six) ; or it may belong to the hyoid arch, thus indicating that the latter 

 is primarily double ; or, again, it may possibly correspond to a branchiostegal ray. 



H 



