Considerable doubt having been expressed with regard to the occurrence of the zone of Ammonites jurensis in the neighbourhood of Northampton, much interest attaches to the discovery, by Mr. T. Jesson of Great Houghton, of the typical Ammonite in some abundance in this area. It is perhaps remarkable that there should have been any doubt about the matter, seeing that in Samuel Sharp's earlier paper Am. jurensis was recorded among the fossils from Duston, and this should have stimulated further search when the doubt arose which seems to have caused this species to be omitted from the later paper by the same author, in which some palæontological errors of the earlier paper were said to be corrected.