176 JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



Silurian rocks are reached, we have no record of their existence through 

 the long period covered by the middle and upper cambrian. 



The fauna of the lower cambrian, while it undoubtedly lived in 

 water of very considerable depth, was not oceanic but continental, and 

 Walcott says that "one of the most important conclusions is, that the 

 fauna of the lower cambrian lived on the eastern and western shores of 

 a continent that in its general configuration outlines the American conti- 

 nent of to-day. Strictly speaking, the fauna did not live upon the outer 

 shore, facing the ocean, but on the shores of interior seas, straits, or 

 lagoons that occupied the intervals between the several ridges that ran 

 from the central platform east and west of the main continental land- 

 surface of the time." 



The lower cambrian fauna was rich and varied, but it was not self- 

 supporting, for no fossil plants are found, and the primary food-supply 

 was pelagic. Animals adapted for a rapacious life at the surface, such 

 as the pteropods, were abundant, and they prove the existence of a rich 

 supply of pelagic animals. All the forms are either carnivorous animals, 

 such as medusa3, corals, Crustacea and trilobites, or they are adapted, like 

 the sponges, brachiopods and lamellibranchs, for straining minute organ- 

 isms out of the water, or for gathering up those which rained down from 

 above, and the conditions under which they lived were obviously very 

 similar to those on the bottom at the present day. 



Walcott's studies show that the earliest known fauna had the follow- 

 ing characteristics : 



1. So far as the record goes it consisted of animals alone, and these 

 animals were dependent upon the pelagic food-supply for support. 



2. While small in comparison with many modern animals, they were 

 gigantic in size as compared with primitive pelagic animals. 



3. The species were few, but they represented a very wide range of 

 types. 



4. All the types have modern representatives, and most of the 

 modern types are represented in the lower cambrian. 



5. The habitat was not the bottom of the deep ocean, but the sub- 

 merged surface of a sinking continent, under water of considerable 

 depth. 



Remains of bottom animals are found in rocks below the cambrian, 

 and Walcott believes that while the olenellan fauna adds a little more to 

 our knowledge of the rate of convergence backwards in geological time 

 of the lines representing the evolution of animal life, it also proves, at 



