152 JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



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

 tion 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 palaeichthyes have preserved their ancient struc- 

 ture 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 differed 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 for crush- 

 ing and grinding the hard-shelled molluscs and Crustacea and echino- 

 derms of the bottom. 



It is true that the sawfish is not confined to the bottom, and the 

 devil-fishes often capture their prey at the surface. In the West Indies 

 they are often found very far from land, but these cases are exceptional, 

 and the true rays rarely leave the bottom, nor are they adapted for rapid 

 movement through the water. 



The rays are undoubtedly much more modern than the true sharks, 

 but there is ample evidence that they have retained habits of life which 

 are common to all the primitive elasmobranchs. 



Many of the modern sharks live on or near the bottom, where they 



