1846.] PRESTWICM ON THE ISLE OF WIGHT TERTIARIES. 231 



organic remains is in the top beds of this group. At Alum Bay 

 these clays are succeeded by about 150, and at White Cliff Bay 

 by about 200 feet of sands, in which no organic remains have yet 

 been found at either place. 



We next arrive at a series of considerable interest formed at 

 Alum Bay, or rather Headon Hill, by a group of green marls and 

 earthy limestones, about 150 feet thick, containing in its lower part 

 but very few fossils, apparently only a few Cytherea incrassata ; but 

 in the upper part it is full of Planorbis obtusus, Limncea fusiformis 

 and Melanopsis carinata, and the other shells of the so-termed lower 

 freshwater formation. 



At White Cliff Bay this series attains a thickness of about 300 feet, 

 and is characterized by singular groups of fossils. In the lowest bed 

 we find the Valuta spinosa, and another species of Valuta, Ostrea fla^ 

 bellula, Natica sigaretina, Pleurotoma colon, Cytherea incrassatay 

 CalyptrcBa trochiformis, and other London clay and marine species, 

 associated with the Cyrena obovata^ Fususlabiatus, Melania fasciata, 

 Neritina concava, Potamides acutus, and Paludina lenta, species 

 common to many of the overlying beds 



Next follow strata Nos. 23 and 24*, containing fewer marine and 

 more estuary and freshwater shells, but still with the same charac- 

 teristic species of Voluta, Turritella, Fusus, Cyrena, Potamides and 

 Paludina. 



The marine species gradually disappear, and we find stratum 

 No. 26 characterized by the Melania fasciata and Cyrena obovata. 



In Nos. 27 to 30 occur the Potamides acutus, Cyrena obovata^ 

 Paludina lenta, and Potamomya plana. 



And in the marls and limestones, Nos. 31 to 35, ending the series, 

 the organic remains are almost, if not altogether, freshwater; they 

 consist almost entirely of several species of the Limncea and Pla- 

 norbis. 



At Headon Hill this division is succeeded by about twenty feet 

 of marl (No. 60. PI. IX.), with rounded flint pebbles, and contain- 

 ing 



Fusus labiatus. Neritina concava. 



Melania fasciata. Melanopsis fusiformis. 



Potamides or Cerithium, several sp. Cyrena obovata. 

 Ostrea. Corbula. 



Natica or AmpuUaria. Nucula, — and many others (see 



p. 246). 



The following characteristic species will serve, I think, to identify 

 stratum 36, at White Cliff Bay, with this so-called upper marine of 

 Headon Hill. Here also the beds are slightly conglomerate. 



An Ostrea — tbe same as that Melania fasciata. 



which abounds at Colwell Bay. Neritina concava. 



Potamides cinctus. Melanopsis fusiformis. 



These are the most abundant species in these strata, but many other 

 species occur, of which a large portion are yet undescribed. 



At Headon Hill this bed is overlaid by a series of nearly 200 feet 

 of marls and earthy limestones, containing principally the 



