v.] 



THE COMMON FROG. 



b7 



markable since the most nearly allied class, that of 

 fishes, is much more rich in salt-water than in fresh- 

 water forms ; while even existing Reptilia have (in the 

 true sea-snakes and in chelonians) representatives 

 which inhabit the open ocean, while in the secondary- 

 geological period marine reptiles {Ichthyosauri and 

 Plesiosauri) abounded. 



Fi::. 24. — Skeleton of the Ichthyosaurus. 



Fig. 25 — Skeleton of the Plesiosaurus. 



Indeed, of all classes of vertebrate animals, this 

 aquatic class {Batrachid) has the least to do with the 

 ocean, for many birds, and a still greater number of 

 Mammals {e.g. the w^hales and porpoises), are constant 

 inhabitants of salt water. All the adult Batrachians 

 feed on animal substances, generally small worms, 

 insects, or slugs and animals allied to slugs. The 

 larger Frogs and Toads will, however, as has been 

 said, devour vertebrate animals, such as mice and 

 small reptiles and birds. The existing large, tailed 

 Batrachians devour fishes. The extinct tailed Batra- 



