xin. 2-3 FOSSIL AMPHIBIA 359 



The skull shows the full series of bones that we have already dis- 

 cussed; there is therefore no reason to suppose that these animals 

 represent a secondarily 'degenerate' branch, which had returned to 

 the water. They were probably very close to the ancestors of all 

 tetrapods. They were numerous in the Carboniferous swamps, but 

 disappeared early in the Permian. So close were these Embolomeri 

 to the ancestry of the reptiles that many workers classify them near 

 the Permian * Seymour ia> which we shall consider as a cotylosaurian 

 reptile (p. 386). 



Throughout the Carboniferous, Permian, and Triassic there were 

 abundant amphibia of partly terrestrial habit, the Rhachitomi, in 

 which both vertebral centra were present, the pleurocentrum being 

 the larger. *Eryops (Fig. 211) was a typical form living in the Permian, 

 about 250 million years ago. The animals were 5 ft or more in length, 

 rather like crocodiles, relatively shorter in body and tail than *Eogy- 

 rinus and with stronger limbs. Nevertheless they probably lived partly 

 in the water and may have been fish-eaters. The skull was long and 

 narrow in the front and short in the 'table' behind the eyes, continuing 

 the previous tendency. A characteristic feature of later labyrinthodonts 

 now began to appear, namely a dorso-ventral flattening of the skull. 

 The pectoral girdle was no longer attached to the skull, but there was 

 a joint between the pelvic girdle and the sacrum. Some Permian 

 Rhachitomi became still more completely terrestrial than *Eryops, 

 for instance *Cacops had very large limbs and protective plates along 

 its back. 



3. Aquatic Amphibia of the later Palaeozoic 



Other lines of amphibia, however, show an accentuation of the 

 tendency to return to the water. In the vertebrae the anterior hypo- 

 centrum became large, while the pleurocentrum disappeared. At the 

 same time the skull became very flattened and the limbs weak. Several 

 stages are known leading from the Rhachitomi to these fully aquatic 

 forms of the Trias, which are placed in the suborder Stereospondyli. 

 *Capitosaurus and *Buettneria are typical of the group, which 

 remained numerous until near the end of the Triassic period, about 

 150 million years ago. Probably this change from rhachitomous to 

 stereospondvlous condition occurred on several independent lines of 

 descent. Thus amphibia, after becoming semi-terrestrial in the Car- 

 boniferous and then probably giving rise to the early reptiles, later 

 returned to the water. 



We also have record of various other secondarily aquatic amphi- 



