The Use of Gryphma in Correlation. 



263 



more than half a whorl and occasionally more than three-quarters, 

 while the Gryphseas of higher zones may rarely be less curved than 

 three-quarters of a whorl, but are generally a whole whorl or more. 

 In other words, some specimens^ perhaps five per oent, in the lower 

 Vermiceras sab-zone are as ad^'anced as those in the gmuendense 

 sub-zone, but the majority of specimens at each horizon illustrate 

 the trend in the evolution of the group. 



Cjryf>ha^ incur\/a. 



? Omuen dense 



Zon« 



Qryphaa sp- 



rotiformis 

 zone 



\ Qryphaea. obUquaka. 

 I y I ? rot/formis zone 



Qryf>hoea sp- 



Vernn'ceras zon^ 



Oitrea I irrequ la ris ' 



Lok«>e<'| un^u/atoL zone 



Number of whorls 



A. 



1 



li 



Fig. 5. — Graphs based on collections of Rryphfeate specimens from several 

 horizons to show the proportions of specimens at each horizon in the 

 various stages of coiling. 



\_For " zone " read " sub-zone."] 



When using Gryphcea in the field for determining the zone it is 

 advisable whenever possible to record the coiling of not less than 



