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
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