TKAKSACTIONS OF SECTION C. 517 



Headon Series. 170 feet xhick. 



Best seen at Headon Hill, Colwell Bay, and at Whiteclift'Bay, and their lowest 

 divisions at Hord well. Everywhere Planorhis eMOJ/ip^aZws characterises the fresh- 

 water bands. 



Potamomya plana and Cerithium 2)Seudocin<:tuin abound in the bractish-water 

 beds. Cytherea {Venus) incrassata, accompanied by many shells, occurs in the 

 marine division. 



The group may be divided mto three sections: Upper, Middle, and Lower 

 Headon. 



Upper Headons. — These constitute the greater portion of the Upper FresJnvater 

 series. The mass of freshwater limestone in Headon Bill belongs to this section. 

 Brackish in the upper part, abounding in Tvtamomya and Cyrena ohovata, at Cliff 

 End they contain a Cyrena (like C. pulchrn). Cerithium trizonatum occurs here 

 abundantly, and Bulimus politus and Melanin niuricata ahovmA. 



Middle Headons. ' The Headon intermarine or Upper marine formation.^ — At 

 Headon Hill these deposits were deposited under brackish-water conditions, for, 

 although Ostrea, Cytherea incrassata, Nucula deltoidea, Natica depressn, Buccinum. 

 lahiatum, and other sea-shells are common, the upper and lower beds abound in 

 Cerithium ventricosum, Cei-ithium concavum, Cerithium pseudocinctum, Neritina ccm- 

 cava, Nematura, l^-c. which are brackish or estuarine. A short distance further 

 north, in Oolwell Bay, the upper and lower beds contain brackish-water shells ; but 

 the central part assumes a distinctly marine character. Ostrea velata, S. Wood, is a 

 characteristic species with numerous marine genera, many of which are of Barton 

 types. This central part is known as the ' Venus bed,'' from the presence of 

 Cytherea incrassata. The marine character of the Middle Headon beds is still more 

 strongly marked at Whiteclift'Bay (22 genera). The lower portion of this series 

 at Whiteclift" Bay contains many Brockeuhurst species, but at Colwell Bay we have 

 no evidence of characteristic species from this last horizon, or in the western side 

 of the island. 



Loicer Headons, fresh and brackish-water series. 



These beds are 70 feet thick in Totland Bav, and 40 feet thick in Whitecliff 

 Bay. 



They consist of fresh and brackish-water beds abounding in fossils resembling 

 those of the upper division. Unio Solandri and Cyrena cycladiformis occur here 

 and are characteristic. 



At Headon Hill the thick bed of limestone in the Upper Headon is conspicuous 

 in the clift's, but it thins out rapidly towards the north and disappears in an easterly 

 direction. The Lower Headon contains a much less thick limestone at Headon Hill, 

 and it is represented by tlie band forming How Ledge between Colwell and Totland 

 Bays. 120 species have been obtained from the Headon series : 104 mollusca, 9 

 Crustacea, 4 annelids, and 3 plants. Land, freshwater, and marine the fossils of the 

 Headon fluvio-marine series, are l\ gasteropoda, '§ pelycipoda; polycipes 1, bala- 

 nus 1, Crustacea |, plantse |, hsh | (the upper figures denoting genera, the lower 

 species). 



The St. Helen's Beds, or Osborne and St. Helen's. 



Between the Bembridge limestone and the brackish-water beds with Potor 

 momyce, that terminate the Headon beds, a considerable series of strata intervenes, 

 which on accomit of their mineralogical and palseontological peculiarities, deserve 

 to hold an intermediate position between the middle and upper Eocene strata, and 

 have been named the Osborne Series. Among the fossils are Paludijice {P. lenta), 

 Melanice (M. costata, M. excaiiata), Melanopsis hrevis and M. carinata. Chara 

 Lyellii is the Gyrogonite of the limestone band, which on the east side divides the 

 Upper or St. Helen's sands from the Lower or Nettlestone beds. The difference 

 between the upper and lower portions of them is considerable. 



Osborne Series in Whitecliff Bat. 



Thickness 100 feet. — Dark red clays and bright red and variegated clays occur. 

 {Helix occlusa, Planorhis discus, and Lymncea lonyiscata.) 



