T he introduction of the Oligocene stage into our classification has necessitated a partial revision of the grouping of our older British Tertiaries. Whether this introduction of a new primary division into the Tertiary system was necessary or expedient may still be questioned; but it has been generally adopted and is, for the time being, established. The division does not coincide in England with a marked change in either fauna or flora, though the series seems nevertheless tolerably complete and well developed; its limits, however widely stretched, show that the Oligocene stage compares neither with the Eocene nor the Miocene in importance. Opinions have differed as to where the line of division should be drawn; whether this should be as low down as the top of the Barton Beds or at the base of the Headon Beds, or even higher. For our part, we think it desirable to uphold the view which places the demarcation between the top of the Bagshot Sands of Alum Bay and the base of the Lower Headon Series, though it is perfectly obvious that any such line in the midst of our series must be a purely artificial one. An almost necessary consequence of the change in the classification has been the readjustment of the divisions of our truncated Eocene, only the middle and lower division remaining, so that the term “Middle” without, an Upper Eocene would no longer be an appropriate one. But the Middle Eocene, embracing as it does such formations as the Nummulitic