414 



T. 0. Bosicorth — Zones of tlic Loircr Clialh. 



The Ecliinoids which are not found in the Bnrwell Rock are 

 Cidarls Bowerhanliii, C. hiruclo, Echinoci/phus dilficilis. and ITolnster 

 treceiisis. The first and last of these are abundant. A large variety 

 of Discoidea ci/Uudiica is also common, and seems distinct from 

 the smaller variety recorded from the beds below. Tlie teeth of 

 rti/i'hodn.t dccnrrens are common in the Chalk, but very rare in the 

 Bnrwell Rock. 



[The subjoined Diagram (Fig. 1) shows the comparative thickness of 

 the zones under the suggested system. The localities chosen do riot 

 lie very far from a mean straight line. The distances between the 

 cohunns roughly represent the distances between the points on the 

 straight line which most nearly represent the positions of the towns 

 named. Vertical scale is 1 cm. = 30 feet. (The Burwell Rock is 

 indicated in black.)] 



Chilton. Dvxstaisle. Cambku>ge. Stoke Feiuiy. IIvnstakton. 



£. plftia. £. plena. B. plena. B. plena. B. plena. 



35 

 18- 



* Base of Chalk Marl. 



Fio. 1. — Comparative thickness of Zoues of the Lower Chalk uiuler suggested system. 



Thus there seems to be a much more definite faunal change on 

 passing up out of the Burwell Rock than on passing up into it. 

 Now the zone of A. variaus in Southern England contains a very 

 rich fauna, but as we pass north of the Thames the fauna of the 

 Chalk ]Marl becomes smaller and smaller, while the thickness of 

 the Burwell Rock increases until in Cambridgeshire, where the 

 Bnrwell llock has its maximum thickness, the fauna of the Chalk 

 ]\Iarl has dwindled down to 23 species. The explanation is that 

 the Burwell Rock is really the most fossiliferous part of the 

 A. variiins zone, and by placing it iu the JI. stibglohosus zone we 



