230 PALEONTOLOGY 



exceptions) live like fishes : the tadpole-stage of the frog 

 is a familiar example. At the present day they are re- 

 presented only by the primitive and somewhat degenerate 

 newts and salamanders, the highly-specialized frogs and 

 toads, and the equally specialized snake-like Ccecilians of 

 the tropics. The two latter orders are unknown before the 

 Eocene period ; the earliest newt is of Lower Cretaceous 

 age ; and between them and the other fossil Amphibia 

 there is at present a great gap in time, no certain remains 

 of Amphibia being known from anywhere in the 

 Triassic or Jurassic strata. In the Carboniferous and 

 Permian systems there are found a series of Amphibia 

 with complex bony skulls, which resemble those of some 

 fishes on the one hand and those of reptiles on the other 

 far more than those of modern newts and frogs. That 

 they were really Amphibia is indicated by their retention 

 of gill-arches in the throat, and by the fact that their ribs 

 do not meet ventrally in a breast-bone, so that they 

 must have breathed like a frog by swallowing air, not 

 sucking it in. Unfortunately little is known of the limbs 

 of these Stegocephalia, and we have no certain knowledge 

 of the stages through which the fin of the fish, adapted 

 to serve as a flexible oar, became transformed into a 

 limb capable of performing the much more complex 

 movements required by a land-living animal. The type 

 of fin which is most easily comparable with the terrestrial 

 limb is that of Ceratodus, found also in some of the 

 Upper Palaeozoic sharks (Pleuracanthus) and ganoids 

 (Holoptyclmis). In this the fin is supported by a cartila- 

 ginous jointed axis, to which are articulated a series of 



