PRE-CAMBRIAN LIFE 49 



we find a few remains referred to Algae, or seaweeds ; 

 small tests or shells of Protozoa ; burrows and trails 

 similar to those of modern sea-worms ; a few bivalve 

 shells allied to modern Lingulae, but presenting some 

 remarkable generalized characters ; some bivalve 

 and shrimp-like Crustaceans, spicules of sponges, 

 and large laminated forms (Cryptozoon) similar to 

 those already referred to as occurring in the Upper 

 Cambrian ; also certain mysterious markings that 

 are supposed to have been produced by the arms 

 or tentacles of free-swimming animals of various 

 kinds. In these lower beds the Trilobites have 

 nearly or quite disappeared, being represented only 

 by doubtful fragments. The beds of rock, origin- 

 ally sandy or muddy sediments, contain fossils very 

 sparingly, and only in certain layers separated by 

 ereat thicknesses of barren material, as if earthy 

 matters were being deposited very rapidly, or as 

 if animal life was rare on the sea-bottom except 

 at intervals. It has, however, been suggested as 

 possible^ that much of the marine population in 

 those early times consisted of pelagic or swimming 

 animals destitute of any hard parts that could be 



1 By Prof. Brookes, of Johns Hopkins University. 



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