CHAPTER IV 



STEGOCEPHALI OR LABYRINTHODOJ'TS -LISSAMPHIBIA APODA 



Sub-Class I. STEGOCEPHALI or PHRACTAMPHIBIA 



With a consideralle amount of dermal armour, especially 

 on the head. 



The earliest known terrestrial four-footed creatures occur in the 

 Carboniferous strata of Europe and North America. They and 

 their immediate allies, which extend through the Permian into 

 the Upper Trias, are now comprised under the name of Stego- 

 CEPHALi, so called because the whole of the dorsal side of the 

 cranium is covered, or roofed over, by dermal bones (a-Teyo'i, roof; 

 Ke(pa\i], head). That these creatures, of which naturally only 

 the skeletal parts are known, were not fishes, is shown by 

 the typically pentadactyloid limbs ; but to recognise them 

 as Amphibia, and as distinct from Eeptiles, is difficult, especially 

 if the incipient Reptilia, which have sprung from some mem- 

 bers of this Stegocephalous stock, are taken into account. 

 However, they possess either two occipital condyles, or none, and 

 their vertebrae are either pseudocentrous or notocentrous, but not 

 gastrocentrous. Moreover, the whole skeletal organisation is still 

 so ideally generalised, that it is easy to derive directly from it 

 the arrangement prevailing in the Apoda and Urodela. 



The vertebral column always comprises a well - developed, 

 sometimes a very long tail. The vertebrae exhibit three types, 

 two of which are fundamentally distinct, while the third is a 

 further development of the second. 



1. Lepospondylous and pseudocentrous. — The vertebra con- 

 sists of a thin shell of bone surrounding the chorda, dorsalis, and 

 is composed of two pairs of arcualia, which meet each other. 



