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. I 



The Lower Cambrian fossils are distributed through strata^ 

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

 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 tc 

 be free from disturbance and under conditions so favorable that 

 it contains the remains of delicate animals not often found aj 

 fossils. 



While the fauna of the Lower Cambrian undoubtedly lived ir 

 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 livec 

 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 shon 

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

 goons that occupied the intervals between the several ridges tha 

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

 nental land surface of the time." dl 



This fauna was rich and varied, but it was not self-supporting 

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

 pelagic. Animals adapted for a rapacious life, such as the ptero 

 pods, were abundant, and prove the existence of a rich supply 0: 

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

 carnivorous, like the medusae, corals, Crustacea, and trilobites, o\ 

 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 undo 

 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 tht 

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

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

 supply for support. While small in comparison with many moderr 



