. Bulletin of the British Museum (Natural History), Geology. 90 J. E. WHITTAKER & R. L. HODGKINSON They observed a significant decrease in the grade of enclosure in specimens from rocks of Eocene age through to those living today and plotted the rate of change of 'factor E' on a graph (see van der Vlerk & Bannink 1969: fig. 3). Operculina ammonoides (Schroter) and O. bartschi Cushman from the Togopi Formation have been used to test this hypothesis. Our two nummulitids, Nummulites cf. amplicuneatus (Cole) and N. tamanensis (Vaughan & Cole), were also measured although we are aware t
. Bulletin of the British Museum (Natural History), Geology. 90 J. E. WHITTAKER & R. L. HODGKINSON They observed a significant decrease in the grade of enclosure in specimens from rocks of Eocene age through to those living today and plotted the rate of change of 'factor E' on a graph (see van der Vlerk & Bannink 1969: fig. 3). Operculina ammonoides (Schroter) and O. bartschi Cushman from the Togopi Formation have been used to test this hypothesis. Our two nummulitids, Nummulites cf. amplicuneatus (Cole) and N. tamanensis (Vaughan & Cole), were also measured although we are aware that 'factor E' might not be applicable to them since van der Vlerk & Bannink studied only Operculina. Twenty good thin sections of each species, all from sample NB 9452, were measured using the prescribed method, the angles e1 and e2 being measured separately by both of us and the arithmetic mean calculated. Fig. 62 shows the percentage distribution of 'factor E' expressed in histograms (compare with van der Vlerk & Bannink 1969 : fig. 2) and the calculated mean values for the four species. It can be seen that O. bartschi and N. tamanensis give very different and much lower results than O. ammonoides and N. cf. amplicuneatus, which although producing almost identical means, have contrasting histogram patterns. Measurements were also made of three other species of known age and plotted (Fig. 63) together with the four Togopi species. Operculina heberti Munier-Chalmas (12 specimens) from 40t 30. 20 10- cr 30 20 10' 0 30 20- 10- 0 Operculina ammonoides (mean 422) © Operculina bartschi (mean 340) Nummulites cf. amplicuneatus (mean 430). Nummulites tamanensis (mean 250} \J ^-x > 100 200 300 400 500 600 700 factor E Fig. 62 Left: Percentage distribution of factor E in four species of Operculina and Nummulites, sample NB 9452, Togopi Formation. Twenty thin sections were measured in each case. Right: Method of measuring factor E from thin sections of megalospheric
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