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Reversed coiling trend in Gryphaea arcuata
Author(s) -
Burnaby T. P.
Publication year - 1965
Publication title -
geological journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.721
H-Index - 54
eISSN - 1099-1034
pISSN - 0072-1050
DOI - 10.1002/gj.3350040203
Subject(s) - mathematics , radius , order (exchange) , geometry , pure mathematics , computer science , economics , computer security , finance
The law of growth of the left valve in Gryphaea arcuata is investigated, and it is shown that it is possible to define a growth‐invariant measure of tightness of coiling. Recent papers by A. Hallam and G. M. Philip are discussed. The latter author's conclusions are shown to be based upon premises which are untenable. It is here concluded that there is a change in tightness of coiling from the angulata to the gmuendense‐bucklandi horizons at the Glamorgan, Yorkshire, and S. England localities from which Hallam collected his samples of Gryphaea shells. The trend is towards less tightly coiled shells in the higher beds, amounting to an average reduction of a few millimetres in peripheral length at an arbitrary constant radius of 20 mm. There are differences between localities of the same order of magnitude, and it is therefore proper to adopt a cautious attitude towards this result. It is however clear that the pronounced apparent increase in shell coiling originally reported by Trueman in 1922 is simply the direct concomitant of increased size.