AbstractThe law of growth of the left valve inGryphaea arcuatais 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 theangulatato thegmuendense‐bucklandihorizons at the Glamorgan, Yorkshire, and S. England localities from which Hallam collected his samples ofGryphaeashells. 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 pronouncedapparentincrease in shell coiling originally reported by Trueman in 1922 is simply the direct concomitant of increased s