DISTKIBUTION OF SAMPLE SETS OF SPECIMENS BY ZONES 301 



stratum of rock at a single locality. Each of the sets except that from 

 the Hamilton is from a zone of rock above the Hamilton, containing 

 Tropidoleptus and other characteristic Hamilton fossils, but at such a 

 position in the rock section that its exact stratigraphic position is fully 

 established. The association of species in each case is sufficiently large 

 and distinctive to establish the recurrent nature of the fauna. 



Between the zones mentioned no trace of the genus Ehipidomella has 

 been found anywhere in the area studied. 



All the specimens gathered were examined, and all those sufficiently 

 perfect to furnish the measurements needed were tabulated. 



Number of Specimens measured 



The total number of specimens measured was nearly 300. Those fur- 

 nishing both length and width were 277, distributed by zones as follows : 



Specimens 



Hamilton 20 



Ithaca 24 



Van Etten 115' 



Owego 40 



Swartwood , 78 



Measurements and Ratio Indexes 



Each one of these specimens was measured by vernier calipers reading 

 to 1/10 of a millimeter, for (a) its length from point of beak to middle 

 of front margin furnishing index L, and (&) its width across from side 

 to side at place of greatest width giving the index W. 



The ratio between length and width obtained by reducing the value 

 of index W to its percentage of the value of index L formed the ratio 

 index R^. 



One hundred and forty-two specimens of pedicle valves showing the 

 interior and the muscular scar were measured. 



These were from the zones as follows : 



Specimens 



Hamilton 12 



Ithaca 



Van Etten 48 



Owego 26 



Swartwood 56 



For each of these the values of the length (index L^) and the width 

 (index W^) of the muscular scar were obtained, and then the measures 



XXI— Bull. Geol. Soc. Am., Vol. 21. 1900 



