235] J. A. Gardner 37 



These faunas are exceedingly interesting, not only because 

 of their diversity and the remarkable 'development of certain 

 groups, but also because of the light they shed on the physical 

 conditions under which they lived. In the following notes, 

 which are based on the study of compiled tables of both recent 

 and fossil forms, an attempt is made to summarize the proba- 

 ble physical conditions indicated by this study. 



Any attempt to reconstruct bottom conditions in the ancient 

 seas must of necessity be based upon data so meagre and so 

 inaccurate that any hope of obtaining absolute values is vain, 

 and yet it does' seem worth while to occasionally gather the 

 imperfect knowledge available and to try to interpret it. 

 Errors do, to a certain extent, neutralize one another and 

 within certain limits general tendencies and relative values 

 can be given with a very considerable degree of assurance. 



Over 800 species have been determined from the Miocene 

 and Pliocene of Virginia and North Carolina and of these 

 approximately 20 per cent, persist into the recent faunas. 

 Certainly a number so large as this ought to give a fairly true 

 line upon general temperature and bathymetric conditions in 

 the middle and later Tertiaries. 



It may be well to consider the main sources of error before 

 giving the conclusions which they modify. 



A. SOURCES OF ERROR IN THE DATA UPON THE TERTIARY 



FAUNAS. 



1. Errors in determination. 



This is one of the least important. The greater part of 

 the work upon the faunas in question has been done by less 

 than half a dozen students and the same collections, for the 

 most part, have been used for reference. Consequently the 

 determinations, whether accurate or inaccurate, are fairly con- 

 sistent. Furthermore, if two forms are so much alike that 

 there is a question as to their identity, a similarity of envir- 

 onmental conditions is implied, even though the differences 

 may later prove to be specific. 



