T. 0. Bom-orth—Znue^ of the Lower Clialk. 413 



In Berkshire, Oxfordsliire, Suffolk, and Cambridgeshire, where 

 the zone of H. siihglohosns is typically developed, it is my 

 experience that the zone fossil is absent from the upper half 

 of the 80 feet of Chalk above the Burwell Rock. It is 

 associated with A. varians in the Burwell Rock. 



In Lincolnshire and Yorkshire H. suhglohosus is abundant in the 

 zone of A. varinns, but so rare in the beds above that 

 Offnster sphericus is used as the zone fossil in its stead. 



In France A. rotomagensis is used as the zone fossil. 



II. On the True Position of the Bxtrwkll Rock. 

 This bed appears to have been placed in the S. suhglohosus zone 

 for two reasons : — 



(1) It contains occasional H. suhglohosus. 



(2) There is a lithological change on passing from the Chalk Marl 



into the Burwell Rock. 



These arguments are neither of them sound. Regarding the first, 

 we have seen that H. suhglohosus is to be expected in the A. varians 

 zone, and A. varians itself is common in the Burwell Rock. As 

 regards the second, the change is often a very gradual one, and the 

 practice of relying on lithology is only a relic of darker ages. 

 There is quite as great a change on passing up out of the Burwell 

 Rock into the Chalk above. Moreover, grey chalk rock beds of 

 very similar nature are included in the upper part of the A. varians 

 zone of Southern England. The fauna of the Burwell Rock is 

 a large one, while those of the Chalk Marl beneath it, and the 

 Chalk above it, are both small. But, of these two, that of the 

 Chalk Marl is more nearly allied to the Burwell Rock fauna than 

 is that of the Chalk above the Burwell Rock. 



The fauna of the Chalk Marl of Cambridge and Suffolk, as recorded 

 in the General Memoir,^ 1903, together with Nautilus elegans, which 

 I have found at Mill Road, consists of 4 Cephalopods, 2 Echinoids, 

 9 Brachiopods, 7 Lamellibranchs, 2 Annelids, and one fish. And of 

 these 25 species, all save Terehratulina triangularis and Serpida 

 amnulata occur in the overlying Burwell Rock, i.e. 92 per cent, of 

 the species are common to both. 



In the Chalk above the Burwell Rock the fauna as recorded in 

 the General Memoir, together with several species not hitherto 

 recorded (to which reference will be made), consists of — 



6 Cephalopods, of which 5 are recorded from the Burwell Rock ; 



7 Echinoids ,, 3 „ ,, 



8 Brachiopods ,, 8 ,, ,, 

 17 Lamellibranchs ,, 12 ,, ,, 



1 Annelid ,, I ,, ,, 



2 Polyzoa „ „ „ 

 6 Fishes ,, 4 ,, ,, 

 4 Reptiles ,, ,, ,, 



i.e., of t.liese 51 species onl}' 33 are recorded from the Bin-well Rock; 

 i.e., 67-4 per cent, of the species are common to both. 



' " General Memoir on the Cretaceous Kecks of Britain," vol. ii (1903). 



