12 LONDON DISTRICT. 



Woods. Small ammonites of the species Pachydiscus peramplus 

 (Mantell) and Prionocyclus neptuni (Geinitz), the loosely -coiled, 

 flat-whorled Scaphites geinitzi (d'Orbigny), and the round- 

 whorled spiral Heteroceras reussianum (d'Orbigny) are typical 

 of the cephalopoda found in this bed. Remains of sponges 

 are notably common. On account of its peculiar fauna the 

 horizon is known as the sub-zone of Heteroceras , reussianum, 

 the index-fossil being recognized by the flanges which are 

 developed at intervals on the ribs running across the whorls. 1 



In some areas the chalk of this sub-zone is extremely hard, 

 and often conspicuous by reason of the presence of green-coated 

 nodules of phosphatic chalk and grains of the mineral glauconite. 

 When this type of lithology is developed the bed is known as 

 the Chalk Rock; as such it has been recognized in a boring at 

 Egham, in the extreme west of our district. 2 



The Zone-fossil Holaster planus (Mantell), a smooth-tested 

 sea-urchin, with flattened base and gently sloping upper surface, 

 is found in the Reussianum sub -zone as well as in the rest 

 of the Planus chalk. Certain Micrasters have the same range. 

 Throughout the Planus chalk Micraster leskei Desmoulins is 

 found with its ' areas ' perfectly smooth, and is identified by 

 this character and by the presence of a smooth, raised, unjointed 

 ring round the border of the mouth. Micraster prcecursor Rowe, 

 as found in this zone, exhibits a zig-zag line between the two 

 rows of tiny plates along the ' areas' ; these ' sutures ' and the 

 slight inflation of the plates are characteristic of this horizon. 

 Echinocorys scutatus Leske, a sea-urchin with oval outline and 

 flattened base, wherein both mouth and anal openings are 

 situated, is also an abundant fossil. 



The Zone of Micraster cortestudinarium is well exposed near 

 Purley Station, and is rich in fossils ; among them we may note 

 the large, thin-shelled sea-urchin Holaster placenta (Mantell), 

 Micraster prcecursor Rowe and M. cortestudinarium Goldfuss. 

 In both species of Micraster the plates in the ' areas ' are so 

 strongly inflated that they are ' subdivided ' in many examples. 

 A gibbous form of Echinocorys scutatus is typical of this zone. 

 The profusion of polyzoa adherent to shells and sea-urchins is 

 also a noticeable feature. 



On account of its thickness the Zone of M . coranguinum 

 occupies by far the greater part of the total Chalk outcrop. It 

 contains a large number of flints, mostly in bands of large 

 nodules. It occupies much of the north-western outcrop, and 

 in the south-east the area round Dartford and Farningham ; it 

 is also found in the isolated areas shown atLewisham, Charlton, 

 Woolwich, Plumstead, Chislehurst and Erith in Kent, and at 

 Purfleet and West Thurrock in Essex. 



1 See Woods, H., ' Mollusca of the Chalk Rock,' Quart. Journ. OeoL Soc. 

 vol. Hi, 1896, pp. 68-98; vol. liii, 1897, pp. 377-404. 



2 ' Geology of London ' (Mem. OeoL Surv.), vol. ii, 1889, p. 196. 



