ROSS — o:n evolution. 413 



apparent externally, that Order being in this as in the ** vegetative 

 repetition " of vertebrae, the most asymmetrical and specialized of the 

 land Vertebrates, so that the poison bag possessed by some of them, 

 was scarcely needed to make the '* Serpent" the fitting emblem of 

 Evil in every Mythology. In accordance with the general law 

 which I have indicated, this highly differentiated Type does not 

 appear among the early representatives of the Class, not having yet 

 been found earlier than the Cretaceous Period. 



The remains of Fishes, the lowest and earliest, the most nume- 

 rous and the most various or differentiated Class of Vertebrates, are 

 found in rocL^-s of the latest Silurian Epoch (and upwards) , of two 

 Orders, the Selachians or Placoids, and the Ganoids. The first of 

 these, of which the Port Jackson Shark is one of the best living, 

 representatives, was approximately homologically * symmetrical, 

 had an internal cartiloginous skeleton, and was covered externally 

 with shagreen or roughened skin, protected by a spine at each fin, 

 and had teeth consisting of broad bony plates, somewhat similar to 

 those which formed a complete bony external skeleton in the only 

 other then existino' Order of Fishes— the Ganoids. These last were 

 less symmetrical than the Selachians, and although at first the 

 internal skeleton was cartalaginous, yet afterwards in the Devonian 

 the Ooccosteiis, has the internal skeleton osseous in the jaws and 

 vertebras, and afterwards other families have internal skeletons still 

 more ossified. The Coccosteus and Pterichthys Families are re- 

 markable as being the first examples of asymmetry, by the limbs 

 being functionless, the former having no apparent fore limbs, and 

 the latter no hind ones. The Teliosts, or Fishes with true bony 

 skeletons and covered with scales externally, appear for the first 

 time in the Cretaceous Period, and are the predominant Type 

 amono^ living: Fishes. Some existinfij Families have no functional 

 limbs, while others are distorted in the most remarkable manner. 

 Thus the Flounder, as also the Plaice, Halibut, &c., "is twisted 

 half around and laid on its side. The tail too is horizontal. Half 

 the features of its head are twisted to one side, and the other half 

 to the other, while its very mouth is in keeping with its squint eyes. 



* Man being taken as the type. 



