4 PEOCEEDINGS OP THE NATIONAL MUSEUM vor,. 70 



turn by still other forms, and so on till the disturbance had reached 

 its end. If the movement in turn was reversed as when a period of 

 elevation succeeded one of subsidence, so in a reversed order many of 

 the old species much changed in form reappear as the conditions 

 necessary to their existence returned. As a result of the changes 

 these disturbances produced we usually find at those horizons where 

 the strata record a considerable change in depth an unusually rich 

 and varied fauna. These species, ranging from the deep-water 

 forms through all the intermediate types to those restricted to the 

 turbulent water of a shallow sea, may follow each other in such 

 rapid succession that all maj'^ be collected in a vertical range of a 

 few feet. 



What should also be referred to in this connection is the remark- 

 able fact that it was only during these periods of oscillation that 

 new species made their first appearance in the Richmond seas and 

 that these horizons where the changes were considerable, the begin- 

 ing of the change is marked by the presence of exceptionally heavy 

 layers of hard, fine-grained clay stone, formed apparently from 

 sediment brought in by currents from a distance. Then it was also 

 during these times of change that numerous old forms which had 

 flourished earlier in the Ordovician seas either during the Trenton, 

 the Cincinnati proper, or earlier in the Richmond reappear to be for 

 a time a part of its fauna. Although, not infrequently, species long 

 established in the Richmond seas degenerated, grew scarce, and finally 

 wholly disappeared during a period of prolonged stabilit}^ we would 

 repeat that as far as our observation has gone, no new species ap- 

 peared or old species reappeared in these waters at any other time 

 than during the periods of disturbance caused by the rising or sink- 

 ing of the sea floor. 



Another interesting fact relating to the fauna that should be 

 mentioned before we take up the consideration of the Richmond 

 strata in detail is the marked effect these changes in depth had upon 

 the size and form of the individuals of such species as passed through 

 them. Notable among the species that were able to survive extreme 

 variations of this nature is the Ra^nesquina altemata and to a less 

 degree the Hebertella occidental'/s group and the Platystrophias. 

 Taking the changes in these forms as an index, it can be stated as a 

 general rule that brachiopods which grew in shallow turbulent 

 waters developed a much thicker shell than those in deeper stations, 

 their brachial valves were more highly arched, the lines of growth 

 more frequently present and strongly developed, and there was a 

 marked tendency to a strengthening of the hinge line much beyond 

 the average. On the other hand in deep water these species devel- 

 oped much flatter and thinner shells than the average, had few if 

 any lines of growth, but frequently became unusually elongated along 



