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 A HISTORY OF BEDFORDSHIRE The Totternhoe Stone has yielded at its headquarters numerous organic remains. Of these it may suffice to mention the crushing teeth of the fishes Ptychodus po/ygyrus, P. latissimus, P. decurrens, and P. mam- milaris, the cephalopod Nautilus atlas, and the lamellibranch Pecten orbicu- laris, specimens of all of which are in the museums of Luton and St. Albans. Another exposure occurs in the Midland Railway cutting at Chalton, where it is visible on the face or the excavation when viewed from the bridge which carries the road to Sundon. This section has furnished Pecten Jissicosta, P. orbicularis, Ammonites varians, and several fine examples of Nautilus elegans, the largest of which, about 8 inches in diameter, is now in the Woodwardian Museum, Cambridge. Overlying the Totternhoe Stone is a tough grey blocky limestone, the zone of Holaster subglobosus, which in some of its exposures has a yellowish tinge at the base, doubtless due to the presence of iron oxide. In its passage upwards it becomes light grey or white. The lines of deposition are indistinct, and it was evidently laid down under tranquil conditions. Its mineral composition and fossil contents indicate that there was a gradual increase in the depth of the sea during its deposi- tion ; one result of which was that many of the genera of Mollusca which previously existed died out or migrated to other districts and were succeeded by others which were adapted to the altered conditions. The stratum contains a high percentage of lime, this material averaging at least three-fourths of its bulk. There is also present a small amount of silica, both organic and inorganic, the former consisting of sponge- spicules and the latter of siliceous granules. Oxide of manganese is often present in the form of a fine black deposit on the surface of the blocks into which it separates during weathering, as well as distributed unequally through the mass. The fossils of this division of the Lower Chalk are both numerous and interesting. Conspicuous amongst them are the palatal teeth of several species of Ptychodus, and the pointed teeth of Lamna, Corax, and Notidanus, the latter occurring only rarely. The lamellibranchs are represented by species of Inoceramus, Plicatula, and Spondylus, and the brachiopods by Rhynchonella cuvieri, Terebratula semiglobosa, and Terebra- tulina striata. Gastropods are of rare occurrence. Echinoderms are represented by the characteristic Holaster subglobosus, and crustaceans by Enoploclytia. Examples of these fossils are to be seen in the Luton and St. Albans museums. Exposures of the bed are not unfrequent along the line of the Chalk escarpment at Totternhoe, Sewell, Puddle Hill Sundon, Sharpenhoe, and Barton. Overlying the zone of Holaster subglobosus is a soft grey chalk or shaly marl known as the zone of Belemnitella plena, or to use the most modern terminology, Actinocamax plenus. This is remarkably persistent across England from the south-west to the north-east coasts. The presence of a seam of impure limestone at this horizon indicates a marked change in the conditions under which it was deposited, and in explanation of its formation the theory has been advanced that a sub- 18