Nearly ten years ago it was my privilege to describe the British Cretaceous forms of this Family in the Geological Magazine. The Family was seen to be capable of subdivision into at least five well-marked genera, and to these a sixth very singular and remarkable one from Aix-la-Chapelle, and also represented at Blackdown, may be added. It was then that the Family reached its zenith, and in the still later Cretaceous rocks of Europe, as a flame burns brightest near its end, so some species suddenly assumed relatively gigantic proportions, and as suddenly became extinct. A few survived in America down to that much later period, the close of the so-called Cretaceous series on that continent. Only two types lived on to the present day and only one of these has ever since Cretaceous times been represented in our area.