I AM extremely obliged to Dr. Newberry for pointing out in a very kind manner what is the actual state of our knowledge at the present time respecting the American cretaceous beds. Never having travelled in America, nor having had the honour of conversing with any of the American savants who have investigated the remains in these beds, I am scarcely in a position to discuss with them the value of the evidence on which they have been considered cretaceous. I have, however, endeavoured to make myself acquainted with the literature of the subject, and had read most of the works mentioned by Prof. Newberry in his letter to NATURE. I in no way dispute that dicotyledonous leaves have or may be found in cretaceous strata, but have, on the contrary, endeavoured partially to account for their absence in British cretaceous rocks. The age of the supposed American cretaceous beds appear to me, however, to be determined principally from the presence of Ammonites, Hamites, and other dibranchiate cephalopoda, and other types of mollusca as Inoceramus. Now what I intended to imply was that the presence of these is not conclusive evidence that the beds in question are as old as our chalk. Between our chalk and the base of our eocene a great hiatus exists, during which almost the whole of the cretaceous fauna became extinct, at least in European seas. This extinction and complete change of fauna implies an immense interval of time which, although we have but little record of it in Europe, we may expect to find recorded elsewhere.