BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA 

 Vol. 21, PP. 295-312 June 25, 1910 



PEESISTENCE OF FLUCTUATING yAEIATIO:NTS AS ILLUS- 

 TEATED BY THE FOSSIL GENUS EHIPIDOMELLA^ 



BY HENRY S. WILLIAMS 



(Presented in abstract before the Paleontological Society Dec. 31, 1909) 



CONTENTS 



Page 



Introduction 296 



Inadequacy of vague descriptive terms 296 



Application of biometric method 297 



Prerequisites to the study of evolution of fossils 297 



Faunal continuity 297 



Environmental change sufficient to permit evolutional modification 297 



Effect of geographic distribution 298 



Artificial selection of evidence 298 



Biometric method 298 



Location and geologic horizon of specimens 298 



Geologic section 299 



The zones and intervals 299 



Hamilton zone 299 



Ithaca zone 299 



Van Etten zone 299 



Owego zone 300 



Swartwood zone 300 



Recurrent Tropidoleptus fauna in the Watkins Glen-Catatonk folio 300 



Distribution of sample sets by zones ,• 300 



Number of specimens measured 301 



Measurements and ratio indexes 301 



Reasons for selection of Rhipidomella for study 302 



Form of shell, ratio index R^ 302 



Muscular scar of pedicle valve 303 



Proportionate rate of growth of parts 303 



Ratio indexes R\ R^, R^ 303 



Method of procedure .'^,03 



Results 304 



Explanation of the ratio indexes 304 



1 Manuscript received by the Secretary of the Geological Society April 23, 1910. 



(295) 



