On the Teeth of Ptychodus and their Distribution in the English Chalk

Abstract

Introductory Remarks. A mong the remains of fishes found in the Chalk, the teeth of Ptychodus are so conspicuous and so easily recognized by the quarrymen that they have long been collected in large numbers and distributed to various museums. Most of these fossils, however, bear no record of the exact locality or zone from which they were obtained, while groups of associated teeth have often been scattered without any note of the circumstances of their discovery. I have, therefore, devoted much attention during the past twenty years to the careful collecting of teeth of Ptychodus from the chalk-pits of the South-East of England, and I now propose to discuss their classification and the zonal range of the various species. I have collected especially in the Gravesend, Rochester, and Medway-Valley area, where there are great excavations in a continuous series of Chalk-deposits, from the base of the zone of Ammonites rhotomagensis to the top of that of Micraster coranguinum . I have also obtained specimens from the Caterham Valley, Oxted, Merstham, and Betchworth; from Berkshire, Hampshire, Bedfordshire, and Cambridgeshire; and I have studied all the material in the British Museum, the Museum of Practical Geology (Jermyn Street), and the museums of Salisbury, Brighton, and Rochester. I have been particularly fortunate in discovering fifty sets of associated teeth, and have examined many more in the museums just enumerated. The arrangement of the teeth of Ptychodus in the mouth is already known from specimens of Pt. decurrens found in the The arrangement of the