146 



GEOLOGY OF THE ISLE OF WIGHT 



The mollusca of the freshwater limestones are nearly all 

 Limnseids belongino; to the genera LimncBa and Planorhis, 

 Limncea longiscata (Fig. 45), and Planorbis cuomphalus (Fig. 44), 



Fig. 44. 



Planorbis euomjjhalus, Sow. 



Fig. 45. 

 Limncea loiigiscata, Sow. 



being perhaps the most abundant and conspicuous species. 

 Paludina lenta (Fig. 46) is a very abundant species throughout 

 the Oligocene Beds, especially in the fresh- 

 FiG. 46. water clays and marls. Nematura -parvula 



Paludina lenta. Sow. is very plentiful, and more generally dis- 

 tributed than is often thought, for its small 

 size causes it to be overlooked. There is 

 also a considerable number of species of 

 land-shells scattered through the lime- 

 stones, but these are not so often met with. 

 They however point to the close proximity 

 of the shore. 



Of other fossils the most commonly found 

 fire valves of Balamis ungviformis in the 

 marine beds, and nucules of Chara, generally C. Wrightii (Fig. 

 47) in almost any part of the series, but especially in the 



iVenfma-bed at the base of the Middle 

 Headon beds. Vertebrate remains are 

 comparatively scarce. Except Chara, 

 there are few recognisable plants. 



Like the other Oligocene beds, the 

 Headon Series seems to be mainly of 

 lagoon or estuarine origin. In the 

 Middle division we have truly marine 

 beds, but these are interbedded with 

 others deposited in brackish water. The Upper and liower Headon 

 Beds are mainly fresh, or brackish-water deposits, and there seems 

 to be an entire absence in them of purely marine genera, such as 

 Valuta, Ancillaria, Pleurotoma, Natica and Cytherea. 



Fig. 47. 

 Chara Wrightii, Forbes. 



