The west of the Island is much the best locality for the lower strata, those which take their name from Headon Hill between Alum and Totland Bays. There are three divisions of the Headon strata, marine beds in the middle coming between upper and lower beds formed in fresh and brackish water. Light green clays are very characteristic of these beds, and at the west of the Island thick freshwater limestones, which have died out before the strata re-appear in Whitecliff Bay. The strongest masses of limestone in Headon Hill belong to the Upper division. The limestones are full of freshwater shells, nearly all the long spiral Limnaea and the flat spiral disc of Planorbis, perhaps the most abundant species being _L. longiscata_ and _P. euomphalus_. The limestones themselves are almost entirely the produce of a freshwater plant _Chara_, which precipitates lime on its tissues, in the same manner as the sea weeds we call corallines. On the shore round the base of Headon Hill lie numerous blocks of limestone, the debris of strata fallen in confusion, in which are beautiful specimens of Limnaea and Planorbis. The shells, however, are very fragile. The marine beds of the Middle Headon are best seen in Colwell Bay, where a few yards north of How Ledge they descend to the beach, and a cliff is seen formed of a thick bed of oysters, _Ostrea velata_. The oysters occupy a hollow eroded in a sandy clay full of _Cytherea incrassata_, from which the bed is known as the "Venus" bed, the shell formerly being called _Venus_, later _Cytherea_, at present _Meretrix_. The marine beds contain many drifted freshwater shells as Limnaea and Cyrena. The How Ledge limestone forms the top of the Lower Headon. It is full of well-preserved Limnaea and Planorbis. The Upper and Lower Headon are mainly fresh or brackish water deposits. The purely freshwater beds contain _Limnaea_, _Planorbis_, _Paludina_, _Unio_, and land-shells. In the brackish are found _Potamomya_, _Cyrena_, _Cerithium_ (_Potamides_), _Melania_ and _Melanopsis_. _Paludina lenta_ is very abundant throughout the Oligocene. A large number of the marine shells of the Headon beds are species also found in the Barton clay. _Cytherea_, _Voluta_, _Ancillaria_, _Pleurotoma_, _Natica_ are purely marine genera. In White Cliff Bay the beds are mostly estuarine. Most of the fossils are found in two bands, one about 30 ft. above the base of the series, the other a stiff blue clay, about 90 feet higher, which seems to correspond with the "Venus Bed" of Colwell Bay. Many of the fossils are of Barton types.