98 THE TRANSFORMATIONS OF THE ANIMAL WORLD 



luxuriant expansion, no longer had strength to coil 

 themselves and thus formed less complicated parti- 

 tions. The first stage belongs to the Primary epoch, 

 the second is Jurassic, and the third Cretaceous. 



The evolution of the Bony Fishes offers us, accord- 

 ing to Gaudry, remarkable stages. At the outset 

 there are strata where no remains of the Pishes are 

 found. This is the commencement of the Primary. 

 A little later, in the Devonian epoch, fishes are ob- 

 served devoid of spinal column or at least in whom 

 this column is soft and formed of embryonic tissue. 

 A little further still, at the end of the Primary, the 

 fishes have a vertebral column incompletely ossified, 

 the arcs of the vertebrae being bony, while the centres 

 are not yet so. At the commencement of the 

 Secondary, the centre of the vertebrae is partly 

 ossified ; in the middle of the Secondary, the 

 geological strata contain, together with species with 

 the substance of the vertebrae not ossified, others 

 having it partly so and others again having it 

 completely so. Finally, when strata are found 

 where the ossification of the vertebras is completed, 

 we may guess that we are at the end of Secondary 

 times, or at a more recent epoch. 



This continuous progress in the ossification of 

 the internal skeleton of the Fishes is, according to 

 Gaudry, correlative to a converse development, that 

 is to say, to a progressive reduction of the ex- 

 ternal one. In the first Fishes, in the middle of 

 Primary times, the body is covered with very hard, 

 large bony plates, which have earned for these 

 animals the name of Placoderms. When we see fishes 

 appear with scales thick, bony, and covered with 



