LONDON CLAY. 27 



fragments, several species of Pleurotoma, Voluta, and Natica. 

 The bivalves include species of oysters, wing-shells (Pteria, 

 sometimes known as Avic-ula-), the straight, tapering, thin-shelled 

 Pinna, and Teredo antenautce J. Sowerby, a burrowing shell that 

 is often found in fossil wood. Fruits of Nipadites also occur. 



In the higher beds fossils are more abundant , as at Kingston 

 Hill, Highgate, Chingford, and Roehampton. Among the bi- 

 valves found are the mussel-like Modiola simplex J. de C. Sowerby 

 and M . elegans J. Sowerby, the latter being smaller, more inflated, 

 and ornamented with fine radial ribs, Meretrix suessoniensis 

 Watelet (= Cytherea tenuistriata J. de C. Sowerby), an inflated, 

 ovate shell, with fine concentric growth-lines, Pectunculus decus- 

 satus J. Sowerby, a small suborbicular shell, ornamented with 

 concentric growth-lines as well as radial lines and finer transverse 

 striae, the small Protocardia nitens (J. Sowerby), marked by 

 several lines of radiating ribs at one end and concentric markings 

 over the rest of the shell, and the large Cyprina planata (J. de C. 

 Sowerby), an inflated species with well-developed umbones. 

 The univalves include Valuta nodosa J. de C. Sowerby, a shell 

 with an elevated spire and blunt spines on the angle of the whorls, 

 and Pleurotoma teretrium Edwards, with a high spire, long canal, 

 a narrow depressed area by the line of junction of the otherwise 

 convex whorls, but with variable ornamentation. Small, flat 

 shells of the brachiopod Lingula tenuis J. Sowerby, thin, narrow, 

 and long, are also found, and at some localities Ditrupa plana 

 J. Sowerby again occurs. A list of some 40 species of fossils 

 from the Bracknell district, to the west of our area, is given in 

 the Windsor Memoir 1 ; there the London Clay is succeeded 

 immediately by typical Bagshot Sand, with a slight unconformity 

 between the two beds instead of the gradual passage. At this 

 locality Mr. A. Wrigley has drawn attention to two fossiliferous 

 horizons at the top of the London Clay ; the higher is a Modiola- 

 bed, and below it is a Cyprina-bed. 2 



While the formation consists of mud brought down by a large 

 river and deposited under estuarine and marine conditions in 

 deeper water than the Woolwich Beds, the fossils indicate a sub- 

 tropical climate; the conditions have been compared to those 

 at the mouth of the Ganges at the present day. 3 



Despite the variety of organic remains and the interest 

 attaching to them, fossils are rarely to be obtained in brickyards 

 or shallow openings in the London Clay, except from some of 

 the septaria. Compared with other great clay formations, like 

 the Lias, Oxford and Kimmeridge Clays, the London Clay offers 

 few attractions to the collector. Chemical changes are probably 

 responsible for the rarity of fossils. The decomposition of 

 iron pyrites leads to the production of sulphuric acid, and when 

 this acts on calcareous organisms or nodules the formation of 



1 ' Geology of Windsor and Chertsey ' (Mem. Geol. Surv.), 1915, p. 31. 



2 Proc. Oeol. Assoc., vol. xxxiii, 1922, p. 79. 



3 Hooker, Sir J. D., ' Himalayan Journals,' vol. i, 1854, p. 1. 



* 17412 B 



