172 AMPHIBIA 



length ; these had terrestrial limbs but traces of gills or gill- 

 arches have not been found in them. 



In another Order of Stegocephali we have Archegosaurus, 

 represented by many well-preserved skeletons which reach a 

 length of four or five feet. These have well-developed terres- 

 trial limbs with four toes in front and five behind ; the enamel 

 of the teeth is much folded in the Labyrinthodont fashion; 

 young specimens show traces of gill-arches and the surface of 

 the bones of the skull is marked with grooves which probably 

 contained dermal sense-organs like those of fishes. Arche- 

 gosanrus occurs in the lower Permian of Germany. The third 

 Order of the sub-class, called Stereospondyli, contains the 

 most highly developed Stegocephali ; Labytinthodon itself is 

 one of the latest genera ; it occurs in the Upper Trias of War- 

 wickshire. Some of these creatures were of gigantic size ; the 

 largest called Mastodonsaurus, from the Trias of England and 

 Germany, had a skull which was nearly a yard in length. No 

 evidence of young or larvse with gills has been obtained in 

 connection with these later Labyrinthodonts. 



Transitions from these ancient Amphibians to the modern 

 forms now existing have not yet been discovered. The Laby- 

 rinthodonts seem to have become extinct at the end of the 

 Triassic period, and scarcely any Amphibian fossils are known 

 from this period to the beginning of the Tertiary. That such 

 remains should not occur in the great thicknesses of the marine 

 deposits such as the Oolite and the Chalk is not surprising, but 

 it is difficult to account for their absence from the fresh- water 

 deposits of the Purbeck and Wealden beds. In the Purbeck 

 beds are several fresh-water strata with remains of fresh-water 

 shells, mammalia, trees, and land plants. There is evidence 

 that the ancient forest soil with its vegetation was slowly sub- 

 merged by fresh water, forming for a time at least a shallow 

 lake or marsh, and furnishing it would be supposed ideal con- 

 ditions for Amphibian life ; and yet these deposits have 

 revealed no Amphibian skeletons. As the Purbeck beds form 

 the uppermost strata of the Jurassic formation so the Wealden 

 are the lowest of the Cretaceous ; in ascending order the 

 Wealden deposits come next to the Purbeck. The lower 

 layers of the Weald clay and the Hastings sands beneath it 

 contain abundant evidence of fresh-water conditions, namely 



