Plate I. A rolled flint pebble, having a Choanite as a nucleus, and the remains of an echinus and spine, shell, and coral, apparent on the surface.

Plate II. A longitudinal section of the pebble, showing the structure of the enclosed Choanite.

Plate III. A polished section of an Ammonite, having the septa or chambers filled with variously coloured spar, &c.

Plate IV. Polished sections of two pebbles from the Isle of Wight; in the upper specimen, the transition from opaque flint to cloudy chalcedony and transparent quartz crystals, is beautifully shown; the lower specimen is richly tinted; the dark appearance is derived from manganese.