Ammonites. London: The Natiral History Museum. 159 p by Monks N., Palmer P.

By Monks N., Palmer P.

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Water is a much more rigorous medium than air in this respect, since it is much more dense, and much heavier. At the surface of the sea, water pressure is exactly the same as it is on dry land at sea level, namely I atmosphere, but it increases by I atmosphere with every additional 10 m (33 ft) depth. Solid objects are not greatly affected by water pressure, since they are non-compressible, and rocks and stones on the bottom of the sea have the same shape and volume as they would on dry land. The same holds true for water, which is also virtually non-compressible, and I kg (15 lb) of sea water has the same volume at the bottom of the ocean as it does at the surface.

When the retractor muscles relax, the head springs forwards and water is suclzed back into the mantle cavity through the aperture of the shell. By alternating between these two actions, nautiluses generate what is called a ventilation cycle, continually bathing the gills in fresh water from which oxygen can be extracted. Just as importantly, the jet produced can be used for swimming, The retractor muscles are attached to the shell at certain points where they leave distinctive 'scars'. These scars can be seen in some fossil nautiluses and ammonites, indicating that they may have been able to produce a similar ventilation cycle.

This sometimes occurs in nodules in calcareous clays. c) The body chamber is crushed flat, but the phragmacone is filled with pyrite and remains uncrushed. This is a common mode of preservation in black pyritous shales and clays. d) The body chamber and the phragmacone are crushed flat. This is the commonest mode of preservation in clays and shales. A M M O N I T E SHELL M O R P H O L O G Y Ammonite shells are built to the same basic plan as the shells of living nautiluses, but there are significant differences.

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