168 W. K. BROOKS. 



5. The bottom fauna soon produced progressive development 

 ■among pelagic animals. 



6. After the establishment of the bottom fauna, elaboration and 

 differentiation among the representatives of each primitive type 

 soon set in and led to the extinction of the connecting forms. 



There is no reason to suppose that the first animals which were 

 adapted for preservation as fossils have been discovered, and many 

 of the oldest fossils, like the pteropods, are most certainly the 

 modified descendants of simpler ancestors with hard parts, but it 

 is interesting to note that the oldest fossil fauna which is known to 

 us is an unmistakable approximation to the primitive bottom fauna 

 as I have outlined it. 



Walcott has given the following sketch of the broad general 

 characteristics of the lower carabrian fauna : 



The lower cambrian fossils are distributed through strata which 

 in Washington and Rensselaer counties in New York, are nearly 

 two miles thick, and some of them, at least, were deposited in 

 water of considerable depth. This is shown by the fineness of the 

 sediment and by the perfect preservation of tracks and burrows in 

 soft mud and of soft animals jlike jelly-fishes. These show that 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 some animals which are not found again 

 until we reach a very much more modern period. The fossil 

 medusae of the lower cambrian are so perfect that their identity is 

 unquestionable, yet it is not until the Solenhofen lithographic slate 

 of the Jura is reached in ascending the geological scale, that 

 medusae are again met with ; and corals and lamellibranchs are 

 found in the lower cambrian, although as they are not found again 

 until the 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 out- 

 lines the American continent of to-day. Strictly speaking, the 



