198 THE TRANSFORMATIONS OF THE ANIMAL WORLD 



of Gyracanihus, indicate in the Coal and in the 

 Permian species of still larger dimensions. 



The group of Dipneusta offers us in the order of 

 the Sirenoids facts of the same nature. The earliest 

 genus, the Dipterus of the Devonian epoch, carries 

 on each of the branches of its jaws one large tooth, 

 the surface of which is ornamented with ten enam- 

 elled crests with crenellated edges. These large 

 dental plates are hardly more than a centimetre 

 across in the Dipterus Valenciennesi of the old red 

 sandstone of Scotland. The dental plates, almost 

 identical, which we find in the Coal of England, 

 which have received the name of Ctenodus, measure 

 as much as six centimetres ; the first correspond 

 to a fish of fifty centimetres at most, the second to 

 an animal exceeding a total length of 1.50m. 



The Palaeozoic Amphibians designated by the 

 general name of Stegocephala, by reason of the 

 bony dermic plates which form a perfect cephalic 

 shield, lived from the Carboniferous to the end of 

 the Trias. The group as a whole presents a most 

 regular progression in size, commencing with the 

 small forms of the lower Carboniferous up to the 

 gigantic types of the upper Trias of Suabia. But we 

 are -here dealing with a very abundant group, evi- 

 dently composed of a great number of parallel 

 branches. If we confine ourselves to the single sub- 

 order of the Labyrinthodons, of which the conical 

 teeth are ornamented with deep and meandriform 

 furrows, we may note in the Coal of England the 

 genera Loxomma and Anthracosaurus, of which the 

 skull already attains from thirty to thirty-five 

 centimetres in length, which we may say in passing 



