THE CARBONIFEROUS PERIOD. 193 



Vertebrated animals save fishes only; but the Carboniferous 

 deposits have yielded re- 

 mains of the higher group 

 of the Amphibians. This 

 class, comprising our ex- 

 isting Frogs, Toads, and 

 Newts, stands to some ex- 

 tent in a position midway 

 between the class of the 

 fishes and that of the true 

 reptiles, being distinguished 

 from the latter by the fact Flg 131 _ Tee th of CochHodu contortut. 

 that its members invariably Carboniferous Limestone, Britain, 



possess gills in their early 



condition, if not throughout life ; whilst they are separated from 

 the former by always possessing true lungs when adult, and 

 by the fact that the limbs (when present at all) are never in 

 the form of fins. The Amphibians, therefore, are all water- 

 breathers when young, and have respiratory organs adapted 

 for an aquatic mode of life; whereas, when grown up, they 

 develop lungs, and with these the capacity for breathing air 

 directly. Some of them, like the Frogs and Newts, lose their 

 gills altogether on attaining the adult condition ; but others, 

 such as the living Proteus and Menobranchus, retain their gills 

 even after acquiring their lungs, and are thus fitted indiffer- 

 ently for an aquatic or terrestrial existence. The name of 

 " Amphibia, " though applied to the whole class, is thus not 

 precisely appropriate except to these last-mentioned forms 

 (Gr. amphi, both; bios, life). .The Amphibians also differ 

 amongst themselves according as to whether they keep per- 

 manently the long tail which they all possess when young (as 

 do the Newts and Salamanders), or lose this appendage when 

 grown up (as do the Frogs and Toads). Most of them have 

 naked skins, but a few living and many extinct forms have 

 hard structures in the shape of scales developed in the integu- 

 ment. All of them have well-ossified skeletons, though some 

 fossil types are partially deficient in this respect; and all of 

 them which possess limbs at all have these appendages sup- 

 ported by bones essentially similar to those found in the limbs 

 of the higher Vertebrates. All the Carboniferous Amphibians 

 belong to a group which has now wholly passed away namely, 

 that of the Labyrinthodonts. In the marine strata which form 

 the base of the Carboniferous series these creatures have only 

 13 



