12 



rocks, and continuing l)y successive forms to the pres- 

 ent time. 



These are followed in structural order by the Dip- 

 noi, or Mud-Fishes, animals which Ijreathe by true 

 lungs, as do the Reptiles. The spmal column and the 

 Ihnbs of the Dipn<jans a})proxhnate to those of the 

 most ancient of the (Jrossopterygian Ganoids, and the 

 annuals approach the x\mphibians even more closely 

 than they do the true Fishes. 



These four branches of generalized vertebrates form 

 a coil around a central stem of Ganoids, which, as Icii- 

 THYOPSIDA, lead up to the more highly specialized ver- 

 tebrate classes. 



From this main stem of the Ganoids, in the Carbon- 

 iferous Period, animals with amphibian tendencies be- 

 ti-an to branch, but it was n(jt until a vast interval of 

 time — in the Cretaceous, that the true bony fishes came 

 upon the scene. The whole group of Ichthvopsida, 

 including the Fishes and Amphibians, appears to arise 

 intermediately between the Elasmo))ranchii and the 

 Dipnoans. The Amphibians, although on a higher 

 plane than the Fishes, seem to express the possiljilities 

 of structure residing in a group whose root is between 

 the Leptocardia and the Dipnoans, whilst the fishes 

 lean to the side of the Elasmobranchii. 



Below the Permian, no Sauropslda are known. 

 Branching from the (ranoid stem, they developed in 

 force during the Triassic and Jurassic Periods, and 

 gave rise to modern reptiles and birds. 



The Mammalian stem ])egan to })ud in the Triassic, 

 with Vromatherhiw ^ and at least eleven genera of 

 mammals were in existence before the appearance of 



