HUT. 22 RICHMOND FAUNAL ZONES — ^AUSTIN 3 



intact and in the same position they were in when death overtook 

 them. That these frail forms were thus so fully preserved by reason 

 of the unbroken quiet of the waters about them and not because they 

 were at once buried after death in the rapidly accumulating clay 

 before their soft parts had time to disintegrate is proved b}^ evidence 

 often furnished by their fossil remains in the form of parasitic 

 growths on their surfaces. In many instances after the death of 

 these animals some parasitic species, such as an incrusting bryozoan, 

 established its colony on the empty shell, reached maturity and died 

 to be succeeded not infrequently by a second and different parasitic 

 species which likewise had full time to complete its life cycle before 

 all were finally covered by the accumulating clay. 



Although the oscillations of the Richmond sea floor were many, 

 the extremes in depth to which we have referred were not the usual 

 outcome of these disturbances, since in most instances the changes 

 stopped well within those limits. Yet a change from one to the other 

 of those extremes did occur a number of times during the upbuilding 

 of the group, and occasionally, as we shall see further on, quite 

 abruptly, as measured by geologic time. At the horizons where these 

 oscillations are recorded in the rocks a half-dozen feet in the verti- 

 cal scale will in some instances take one from strata laid down in the 

 quiet of deep water to other strata, whose broken and water-worn 

 fossils indicate that they were formed in a shallow sea whose floor 

 was subjected to the full force of the waves. While the character 

 «nd conditions of the sediments of the Richmond seas were greatly 

 affected by these oscillations, a still greater influence was exerted on 

 its fauna. Tlie species of the deep-water beds are almost all dif- 

 ferent from those met with in the shoal-water beds and so, to a lesser 

 degree, do both differ from the life of the intermediate strata. As a 

 rule, lamellibranchs greatly outnumber all other classes in the deep- 

 sea beds, while in those formed in shallow waters the brachiopods 

 and bryozoans are the predominating forms. Since by far the 

 greater part of the Richmond species appear to have had their range 

 determined by sea depth, and as many of these species, like the 

 Hehertella insculpta., seems to have been unable to survive any con- 

 siderable change in that condition, the oscillations that so frequently 

 resulted in the deepening or shoaling up of the Richmond sea un- 

 doubtedly exerted a powerful influence on the life of its waters. 

 Whether the sea floor was rising or sinking, every step in the change 

 was reflected by a corresponding change in the fauna of the time. 

 Single species, and at times whole groups of species, were forced to 

 retire from the disturbed area since the change had rendered its 

 waters untenable. These disappearing species were at once suc- 

 ceeded by other species adapted to the new conditions and these in 



