HEADON SERIES A.3.
These beds are seen both in Whitecliff Bay, Headon Hill, and Alum Bay, or at the
east and west extremities of the Isle of Wight. The upper and lower portions are
fresh-water, and the middle of mixed origin, sometimes brackish and marine.
Everywhere Planorbis euomphalus, Figure 175, characterises the fresh-water
deposits, just as the allied form, Planorbis discus, Figure 170, does the
Bembridge limestone. The brackish-water beds contain Potamomya plana, Cerithium
mutabile, and Potamides cinctus (Figure 37 Chapter 3), and the marine beds Venus
(or Cytherea) incrassata, a species common to the Limburg beds and Gres de
Fontainebleau, or the Lower Miocene series. The prevalence of salt-water remains
is most conspicuous in some of the central parts of the formation.
(FIGURE 176. Helix labyrinthica, Say. Headon Hill, Isle of Wight; and Hordwell
Cliff, Hants-- also recent.)
(FIGURE 177. Neritina concava, Sowerby. Headon series.)
(FIGURE 178. Lymnea caudata, Edw. Headon series.)
(FIGURE 179. Cerithium concavum, Sowerby. Headon series.)
Among the shells which are widely distributed through the Headon series are
Neritina concava (Figure 177), Lymnea caudata (Figure 178), and Cerithium
concavum (Figure 179).
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