The Charophyta of the Lower Headon Beds of Hordle [Hordwell] Cliffs (South Hampshire)

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Abstract
The correlation of limestone-deposits lying in isolated basins has always been a matter of much difficulty and uncertainty. Thus far no group of freshwater fossils has proved sufficiently widespread and characteristic for the purpose of zonal work. The mammals have a very limited geographical range, their assemblage depends so largely on the nature of the surrounding country, and their remains are usually so scarce, that they do not greatly help ; besides which, in strata older than the Tertiary they almost fail us. Other vertebrates, such as fishes, would also vary greatly in adjoining areas. The freshwater and brackish-water mollusca, which are so abundant, and would at first sight appear so useful, cannot be depended on for the comparison of distant areas. Many of the species are very local, and their variation under slightly changed conditions is so great as to give rise to much confusion. Anyone who has worked with the genera Unio, Corbicula, Cyrena, Paludina, Melania, Potamides, or Limnæa in the London and Paris Basins must have felt this difficulty. In the case of the non-sculptured genera, such as Paludina , the difficulty is increased by a tendency to vary around a certain generic type, to which the species seem t o revert again and again. Indeed, in certain of the molluscan genera the specific differences are so slight as only to be recognizable in specimens better preserved than such as are commonly available. The higher plants are usually very local, and their remains are Q. J. G. S>

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