54 THE COMMON FROG. [chap. 



sizes flitted through the air. The ocean was 

 peopled not by whales and dolphins, these had 

 not yet appeared, but by huge Ichthyosauri and 

 Plesiosauri. Reptiles of huge bulk (Iguanodons, 

 Megalosauri, Notosauri, &c. &c.) fulfilled the parts of 

 herbivorous and carnivorous beasts, and altogether 

 the Mammalian fauna of to-day was represented by 

 analogous reptilian precursors (figs. 24, 25.) 



May it not have been similar in yet older periods 

 with regard to animals of the Frog class .? We have 

 seen the possibility of aerial locomotion in even the 

 existing Rhacophorus. It is true that all existing 

 Urodeles are fresh-water forms, but it may well be 

 that marine creatures once bore the same relation to 

 them as the great marine Ganoid fish fauna bears 

 to the few existing Ganoids^ which now constitute a 

 fresh-water group. 



The great crocodile-like Labyrinthodonts must 

 have been no ignoble predecessors of the rapacious 

 reptiles which were to succeed them, and the fossil 

 form Ophiderpeton suggests that the OphioinorpJia 

 may be the last remnants of a race which preceded 

 and represented the subsequently developed serpents. 



This, however, is but a conjecture which future 

 discoverers will probably ere long establish or refute. 



The name Labyrinthodonta was bestowed upon the 

 great fossil group on account of the beautiful and sing- 

 ularly complex structure of the teeth of some members' 

 of the order. These teeth are conical, and exhibit 

 slight vertical grooves on their surface. A horizontal 



^ Existing Ganoids are the sturgeon, bony pike (Lepidosteus), mud 

 fish (Lepidosiren), and others as noticed earlier. 



