The Upper Greensand of Cambridge occurs as a thin but very important bed, covering the eroded surface of the Gault, and blending upwards into the overlying Chalk-marl. It consists essentially of Chalk-marl, saturated with green grains of Glauconite, and crowded with a variety of fossils. The fossils in great part derive their origin from the Gault, out of which they have been concentrated by a natural process of levigation. At the time when the Gault sea was shallowing to its close, its submarine shores became eroded by a cold current flowing from the north, which bore away its lighter sedimentary clay, and left its imbedded fossils behind, to be subsequently rolled into the sublittoral deposit of the Upper Greensand. Thus it happens that in our neighbourhood the uppermost beds of the Gault are not to be found, and their only representative is this mere pebble-bed at the base of the Chalk. The rich fauna in our formation thus collected ready to the hand of the Palæontologist has been well worked out in its higher developments by Mr. Barret, Mr. Seeley, and various other observers. Of the Protista and Cœlenterata, however, some departments have been left all but untouched; and it is to the Sponges of the latter and the Foraminifera of the former that my paper refers. The Foraminifera occur abundantly, and in a rich variety, which seems to anticipate their more luxuriant appearance in the succeeding Chalk. The Vitrea perforata are represented by large forms of Bulimina, Textularia, and Orthocerina, which mimic truly arenaceous foraminifera by imbedding the siliceous and volcanic sand of the formation in their tests.