142 W. K. BROOKS. 



examination will show that there is ample evidence that it is sec- 

 ondary, and not primitive. 



The perfection of their adjustment to a free life in the open sea 

 is no evidence that this life is primitive, for the highest marine 

 animals and those whose adaptation to a pelagic life is most com- 

 plete, the sea-birds and cetacea and marine reptiles, are air-breathing 

 terrestrial animals which have gone back into the ocean. 



The most primitive groups of living fishes are the cycl9stomes, 

 elasmobranchs and ganoids. The cyclostomes are too small a group, 

 and the living forms are too aberrant in habit, to contribute much 

 information regarding the nature of the primitive vertebrates, but 

 they exhibit no evidence of adaptation to a pelagic life, and our 

 scanty knowledge of them is quite in harmony with the view that 

 their remote ancestors were bottom animals. 



The case is very different as regards the great groups of modern 

 fishes for which the term palaeichthyes is often used ; the sharks, 

 rays and ganoids. 



The living representatives of these great and ancient groups are 

 of peculiar interest to naturalists on account of their close affinity 

 to the oldest vertebrate fossils which have been discovered. These 

 points of resemblance to the more modern, but still ancient, amphibia 

 and teleosts show that the modern palseichthyes have preserved their 

 ancient structure with very slight modification, and that we have 

 in them one of the most remarkable stem forms in the whole animal 

 kingdom. This is shown still more conclusively by the fact that 

 some of the palaeozoic families of elasmobranchs have lived through 

 period after period of geological history and have held their ground 

 up to our own times. 



The abundance and variety of the remains of elasmobranchs in 

 the palaeozoic rocks prove the great development of the group at 

 this remote and early period, and the silurian sharks probably dif- 

 fered but little from those of the present day, although we are forced 

 to see in them the ancestors of the ganoids and of all the divergent 

 groups of extinct and living vertebrates. 



Of the three groups of modern elasmobranchs, two, the chimaeras 

 and the rays, are bottom-feeders. The whole organization of the 

 ray is as obviously adapted for life upon or near the bottom as that 

 of a bird is for life in the air, and the flat pavement teeth are adapted 



