SKULL 



103 



solidarity of the osteocranium, doubtless due to adaptation in 

 connection with the burrowing habits of this group (Fig. 75). 

 The differences are due essentially to the more extensive ossifi- 

 cation, and to the fusion of certain of the bones, e.g. the para- 

 sphenoid, otic, and occipital elements, and the premaxilla and nasal. 

 The temporal region, moreover, is more or less covered by bone, 

 and the ethmoid skeleton is well developed, the solid nasal septum 

 enclosing no gland. 



The post-mandibular visceral skeleton consists in the larva, 

 as in that of (Jrodeles, of a hyoid and of four branchial arches 

 which, however, have a more primitive and piscine form and 

 arrangement and are less reduced than in Urodeles (Fig. 75, D). 

 In the adult, the median basal connection between the first and 



Oc 



FIG. 76. RESTORATION OF THE SKDLL OF A SMALL STEGOCEPHALAX (PROTRITOX) 

 from the Carboniferous of Bohemia. (After Fritsch.) 



Br, branchial apparatus ; F, frontal ; Fp, parietal foramen ; J/, maxilla ; JV, 

 nasal; Na, nostril; Oc, gclerotic ring (orbital bones); P, parietal; Pf, 

 prefrontal ; Pmx, premaxilla ; Socc, supraoccipital. 



second branchial arches disappears, and the vestige of the fourth 

 arch unites with its predecessor. 



The skull of the fossil Stegocephali, some of which were 

 comparatively gigantic, was covered by a much larger number of 

 dense and firm bony shields, and a foramen was present in 

 the parietal region, which, like that in modern Lizards, is in 

 relation to the pineal apparatus (Fig. 76). A row of bony 

 sclerotic plates was usually present around the orbit like those 

 of Birds and certain recent and fossil Keptiles (e.g. Lizards, 

 Ichthyosaurus). Many of these forms possessed the same number 

 of visceral arches as Urodeles, and it has been shown that they 

 (e.g. Branchiosaurus) underwent a metamorphosis. 



The class Amphibia, like the Reptilia, was of much greater 

 importance in earlier periods of the earth's history than at present, 

 but existing forms cannot be traced directly to those of the 

 Carboniferous and Permian strata. 



