ZONES OF THE CHALK. 13 



Fossils are scarce at the base of the zone, but occasional 

 specimens of Echinocorys scutatus Leske and Micraster prcecursor 

 Rowe are found. The ' areas ' in the latter are more advanced 

 in subdivision than is the case in the preceding zone. In the 

 upper two -thirds of the zone the Micrasters reach a stage of 

 development known as the high -zonal type, distinguished by 

 the appearance of a sharp cut between the two rows of plates 

 in the ' areas.' The Micraster prcecursor and M . cortestudinarium 

 of lower horizons are here replaced by the typical M . coran- 

 guinum Leske and the wider M. coranguinum var. latior Rowe. 

 In this upper portion of the zone the frequent occurrence of the 

 conical sea-urchin Conulus albogalerus Leske is characteristic. 

 Similar to Micraster but distinguished in form by its pyramidal 

 shape, the sides of its upper surface sloping steeply from the 

 central apical disc, is Epiaster gibbus (Lamarck), found also at 

 this horizon. 



The Marsupites Zone frequently yields fragments of the 

 fossil from which it takes its name, but is more often recognized 

 by the Uintacrinus band at its base ; it has been traced along 

 the southern outcrop from Ewell to Croydon, Keston, Orpington 

 and so on, and also at the top of the exposures at Charlton. 

 The Uintacrinus band is marked by the occurrence in it of the 

 small arm -joints and irregular, polygonal, deeply -grooved plates 

 of the stemless crinoid of that name ; Micraster coranguinum 

 Leske, Echinocorys scutatus Leske, and Conulus albogalerus Leske 

 also occur in this band. Above is the Marsupites chalk, wherein 

 remains of the stemless crinoid Marsupites testudinarius 

 (Schlotheim), chiefly the six-sided, radiately ridged plates, are 

 found. These are associated with the species previously 

 mentioned, of which Echinocorys scutatus is represented by the 

 variety pyramidatus Portlock. 



Many species of fossils have a wide range in the Chalk zones, 

 although detailed study has shown that variations in form can 

 be correlated with zonal occurrence. Skeletons of sponges are 

 frequently found, the commonest being Porosphcera globularis 

 (Phillips), a small, usually spherical fossil, with minute canal- 

 apertures over its surface ; and Ventriculites, funnel-shaped or 

 cylindrical, sometimes with * root ' attached, and often con- 

 spicuous on account of the light-brown colour of the iron replace- 

 ment. Isolated cylindrical corals of the genus Parasmilia are 

 fairly frequent: the ornament on the fine longitudinal ridges 

 gives indication of the zone. 1 



Spines of the sea-urchin Cidaris, usually rod-like, are recog- 

 nized by the close fret-saw like ridges ; only rarely are they 

 found still attached to the highly ornamented, spherical test. 

 The spines of C. clavigera Koenig are typically club-shaped. 

 Found in the Upper Chalk, Rhyndionella plicatilis (J. Sowerby) 

 is marked by a sharply -folded margin. Terebratula semiglobosa 



1 See Lang, W. D., Proc. Zool. Soc., 1909, p. 285. 



