Tlte Use of Gryi^hcBa in Correlation. 261 



Measurement of the spirals of the Gryphseas studied has shown 

 that the spiral angle a is the lowest in the most primitive members, 

 and increases to about 80° in G. incurva. 



In general it also appears that in the development of a gryphaeate 

 shell the angle a increases in a corresponding manner. In the early 

 stages of G. incurva the angle may be only 60'^, as it is in the adult 

 stages of G. ohliquata. 



Fig. 3. — Diagram of a gryphsean spiral. Spiral angle, o ; origin, o. 



The Use of Gryph^a in Correlation. 



As we have noted, the lineage of G. incurva ranges from the 

 Oyster Beds at the base of the Lias, where it is represented by flat, 

 oyster-like forms, to the bucJdandi zone, where it is typically 

 represented by very incurved forms. Consequently the various 

 horizons of the Lower Lias can be identified from a study of the 

 Gryphseas, even when no ammonites are present. For instance, 

 the Gryphseas have proved of considerable value in correlating the 

 littoral Lias of Glamorgan, in which ammonites are very uncommon. 

 In those cases where ammonites have subsequently been found, 

 they have invoriably confirmed the conclusions that had been based 

 on GryjjJuEa. It is posaible to identify both the zones and the sub- 

 zones in this way. 



Some caution is necessary in deciding the precise horizon of 

 beds with Gri/phcea on account of the variation within the group. 

 Even in the same block of stone they often exhibit marked differences. 

 In a large collection from one horizon it will generally be found 

 that about half the specimens are of similar aspect, with left valves 

 coiled to almost the same extent. Of the remainder, some are less 

 coiled and some are more coiled than the average.^ 



^ Among Liassic Gastropoda, and some families of Ammonites, it is not 

 uncommon to find adult specimens at tlie same horizon at somewhat different 

 stages of evolution ; some are more " highly accelerated " than others. The 

 differences between the Gryphseas, however, are much more apparent. 



