236 THE FOUNDATIONS OF ZOOLOGY 



tion as fossils have been discovered, but it is interesting to note 

 that the oldest known fauna is an unmistakable approximation to 

 the primitive fauna of the bottom. 



The Lower Cambrian fossils are distributed through strata 

 more than two miles thick, some, at least, of them showing by their 

 fine grain, and by the perfect preservation of tracks and burrows 

 which were made in soft mud, and of soft animals like jelly-fish, 

 that they were deposited in water of considerable depth. The 

 sediment was laid down slowly and gently in water so deep as to 

 be free from disturbance and under conditions so favorable that 

 it contains the remains of delicate animals not often found as 

 fossils. 



While the fauna of the Lower Cambrian undoubtedly lived in 

 water of very considerable depth, it was not oceanic but conti- 

 nental, for we are told by Walcott that "one of the most impor- 

 tant conclusions is that the fauna of the Lower Cambrian lived 

 on the eastern and western shores of a continent that in its gen- 

 eral configuration outlines the American continent 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 la- 

 goons that occupied the intervals between the several ridges that 

 ran from the central platform east and west of the main conti- 

 nental land surface of the time." 



This 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, such as the ptero- 

 pods, were abundant, and prove the existence of a rich supply of 

 pelagic animals. All the forms known from the fossils are either 

 carnivorous, like the medusae, corals, Crustacea, and trilobites, or 

 they are adapted, like the sponges, brachiopods, and lamellibranchs, 

 for straining minute organisms out of the water or for gathering 

 those which rained down from above, and the conditions under 

 which they lived were very similar to those on the bottom at the 

 present day. 



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

 following characteristics : It consisted, so far as the record shows, 

 of animals alone, and these were dependent upon the pelagic food 

 supply for support. While small in comparison with many modern 



