T. 0. BosicoHh — Zones of the Lourr Chalk. 



415 



endow that zone with an abnormally rich fauna and impoverish that 

 of the A. varians zone. The fauna of the Burwell Rock is one 

 typical of the zone of A. varians, and is practically identical with 

 that of the shelly beds in the upper part of that zone in Kent. 



Of the 16 Cephalopoda (including Bacnliles baculoides, not 

 recorded in the General Memoir, but which I find common at 

 Fulbourn), 9 Echinoids, 10 Brachiopods, 11 Gasteropods, 25 Lamelli- 

 branchs, and 2 Corals, all occur in the zone of A. varians of Southern 

 England, with the exception of Nautilus rejlectus and 6 Lamelli- 

 branchs, of which I can find no record except occurrence at Burwell. 

 A. varians itself is more plentiful in the Burwell Rock than in the 

 underlying Chalk Marl. 



This figure (Fig. 2) shows the comparative thickness of the zones 

 under the present system : — 



Chilton^ Dunstable. Cambridge. Stoke Ferry. Hunstanton. 

 B. plena. B. plena. B. plena. B. plena. B. plena. 



80- 



3' Bjize_ _ _ Vf_ ^tl2e_ J Bury ell 



JOO' 



SO' 



20' 



It 



yo- 



'Rock 



26 



2' 



80' 



PW' 



/8' 



SO' 



75' / 



* Base of Chalk Marl. 



Fig. 2. — Comparative thickness of Zones under present system. 



If, on the strength of these arguments, the Burwell Rock be 

 relegated to the A. varians zone as a mere facies of Chalk Marl, 

 the thinning of the Lower Chalk towards the north-east is some- 

 what simplified (see Figure), the thickness of the two zones 

 decreasing more uniformly and simultaneously. The most con- 

 venient place to draw the line of separation is the well-marked 

 band of Bhynchonella Mantelliana. This occurs a few feet above 



