W. K. BROOKS ON THE GENUS SALPA. 175 



more important era of rapid development in the forms of animal life has 

 never received the attention which it so well merits. 



If the views which I have advanced are correct, the primitive bottom 

 fauna must have had the following characteristics : 



1. It was entirely animal, without plants, and it at first depended 

 directly upon the pelagic food-supply. 



2. It was established around elevated areas in water deep enough to 

 be beyond the influence of the shore. 



3. The great groups of metazoa were rapidly established from pelagic 

 ancestors. 



4. There was a rapid increase in the size of the bottom animals and 

 hard parts were quickly acquired. 



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 charac- 

 teristics of the lower Cambrian 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 like 

 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 con- 

 ditions 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 

 medusas are again met with ; and corals and lamellibranchs are found 

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



