Additional observations on the evolution of Gryphaea

The view that Gryphaea became uncoiled as it evolved, recently presented by T. P. Burnaby (1965), is shown to be based on the incorrect use of a size dependent parameter. His result is largely the outcome of the larger mean size of the higher subzone specimens in Hallam's collections. When this size dependence is allowed for, then the results of Burnaby's method are similar to those given by Philip (1962). An additional treatment of Hallam's data is given and it is shown that the slopes of the reduced major axes fitted to Hallam's samples by Philip (1962) are an adequate index of the tightness of coiling (estimated by s/r) of the larger specimens in the samples. Limitations of Burnaby's method are briefly discussed.