“In order to arrive at satisfactory results in an undertaking so vast,¹ I shall need the assistance of all persons who are interested in the advance of geology, and I trust to your procuring for me in England many correspondents willing to exchange the fossil shells of various formations for such portions of my works on Palæontology as they may most require. I wish, for instance, to obtain, 1st, fossils of the Crag and London Clay, and 2nd, those of the Cretaceous formations of the Isle of Wight and Blackdown; but, above all, the fossils of your Carboniferous, Devonian and Silurian beds.”