I. Introduction During an examination of the Lower Cretaceous outcrop in Bedfordshire by one of the writers in June 1902, he was delighted to find in the sands immediately below the base of the Gault-Clay, in the large sand-pits at Shenley Hill, 1½ miles north-east of Leighton Buzzard, a richly-fossiliferous horizon which does not appear to have been hitherto noticed. Four separate visits were afterwards made by him to the locality, for the purpose of studying the section and collecting the fossils; but as these did not serve to exhaust the possibilities of the bed, the second author undertook to carry on the investigation, and for this purpose spent ten days on the ground in August. On the material thus jointly collected, along with further supplies since received from the quarrymen, the palæontological results given in this paper are based, while for the stratigraphical details of the sections, and for opinions expressed thereon, the first-named author is mainly responsible. The nearest locality from which fossils have been previously recorded in the Lower Greensand of this part of England, lies between Great and Little Brickhill, 4 miles to the northward of Shenley Hill, where they were found abundantly in working a ‘coprolite-bed’ at the base of the Lower Greensand. The only other recorded occurrence of marine Lower Cretaceous fossils in Bedfordshire is at Potion, 19 miles to the north-eastward of Shenley Hill, where, in the workings of a pebbly ‘coprolite-bed’ which forms a band in the Greensands