SALPA IN RELATION TO EVOLUTION OF LIFE. 143 



for crushing and grinding the hard-shelled molluscs and Crustacea 

 and echinoderius 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 are found in immense numbers and at considerable depths. 

 In 1888 I was invited by Marshall McDonald, the Superintendent 

 of the U. S. Fish Commission, to make use of the opportunity for 

 surface collecting which was afforded by an expedition which was 

 sent out to fish witli hook and line on the bottom and along the 

 edge of the Gulf Stream. The fishing commenced at the 500 

 fathom line, and every time the line was taken in we found 

 numbers of dogfish (Scyllium) on the hooks, even when the water 

 was considerably more than half a mile deep. 



Many genera of sharks, such as the houndfish (Mustalus) and 

 the dogfish (Scyllium), are known to feed upon the molluscs and 

 Crustacea and worms of the bottom, and the flat pavement-teeth of 

 other genera whose habits are less known show that their mode of 

 life is the same. Some of the bottom-feeding sharks (Cestracion 

 for example) are the oldest of living vertebrates. 



The mailed ganoids were undoubtedly derived from a shark-like 

 ancestor, and the structure of the oldest ones, such as perichthys, 

 coccosteus, and cephalaspis, shows that they were not very rapid 

 swimmers. They were, undoubtedly, bottom-feeders like the 

 modern sturgeon, and like many large and important families 

 of modern teleosts, such as the cod, the siluroids and the 

 pleuronectidae. 



So far as we know the palaeozoic waters from fossils, there were 

 no active locomotor animals of large size to furnish prey for 

 raptorial fishes and the existence at the present day of so many 

 species and genera and families of bottom-feeders, and the fact 

 that the most archaic forms have this habit, are all grounds for 



