262 Dr. A. E. Trueman — 



The number of whorls forms a measure of the coiling, and also, 

 m a general way, is an indication of the position in the lineage. 

 A curve may be constructed like that in Fig. 4 showing graphically 

 the proportions of specimens from a particular horizon with the 

 various degrees of coiling. It will be noticed from Fig. 4 that the 

 majority of specimens of G. cf . incur m from the gmuendeme sub-zone 

 of Bridgend are coiled through more than a whole whorl ; a few 

 are coiled for considerably more than a whorl, while others are coiled 

 for only three-quarters of a whorl. The difference in aspect between 

 these two extreme forms is considerable, and it is obviously impossible 

 to determine the horizon represented if only a few specimens are 

 available. 



O- 



/ '\ 



/ \ 



/ \ 



/ \ 



/ \ 



/ V 



number of whorls 



1/^ iVx 



Fig. 4. — Grap]i based on a collection of Gryphcna aff. incurva from a horizon 

 in the gmvendense sub-zone, Bridgend, Glam., to shois? the proportions 

 of specimens in various stages of ceiling. 



When these facts are remembered it is easy to understand the 

 confusion that has arisen concerning the horizons at which various 

 species are found. For example, G. incurva has been recorded by 

 various writers from the jflanorhe zone, the angnlata zone, and the 

 hucJdandi zone, and although many of the records probably refer 

 to specimens that are really less advanced than the true G. incurva, 

 it should be noted that quite curved forms of Gryphcea occur some- 

 what rarely in the angulata zone, and not uncommonly in the lower 

 part of the hucTdandi zone.^ 



In Fig. 5 are shown graphs representing the proportions of 

 specimens in the various coiling stages from several horizons of the 

 Lower Lias. In the lower portion of the angulata zone (liasicus 

 sub-zone) Ostrcea irregularis represents the G. incurva lineage, and 

 rarely shows a curve of more than a quarter of a whorl The 

 Gryphaeas of the Vermiceras sub-zone are typically curved through 



^The very curved forms from the angulata zone generally differ from 

 G. incurva in having a feeble sulcus (if any) and a larger area of attachment. 

 They are, therefore, accelerated in some characters only. 



