510 A. J. Jukes-Browne — Zones of Lotver Chalk. 



fossil." So far as my experience goes it is uot plentiful in either 

 zone, but is occasionally to be found in Dorset near the base of the 

 A. varians zone and near the top of the H. subglobosus zone. As 

 regai'ds Devon I am at a loss to know what he means ! 



Mr. Bosworth also states that If. trecensts "is common throughout 

 the [upper] zone in Hants, Sussex, Kent, and the Isle of Wight." 

 He writes as if this was a well-known fact or could easily be gathered 

 from a perusal of the Survey Memoirs, whereas I do not think that 

 any such statement has been made either in these memoirs or in any 

 other publication, except with regard to the Isle of Wight. In 

 Sussex it would seem to be a scarce fossil, for I was not able to 

 record the occurrence of a single specimen from the zone of 

 JB. subglobosus. 



The only satisfactory and useful part of Mr. Bosworth's article is 

 the record of his own observations near Cambridge. I have no 

 reason to doubt the accuracy of these, and am quite ready to accept 

 his statement that H. trecensts is more abundant than would appear 

 from the lists in the Memoirs of the Geological Survey, and also that 

 it occurs chiefly in the upper half of the zone, while H. subglobosus 

 occurs more abundantly (if not exclusively) in the lower half. 



I believe that the same distribution of these two species prevails 

 in Hertfordshire, Bedfordshire, and Buckinghamshire, as well as in 

 Hampshire, Surrey, and Kent. This is a point which is well worth 

 further investigation, and if their relative abundance proved to be 

 the same as in Cambridgeshire, it might lead to the establishment of 

 the two subzones which are suggested by Mr. Bosworth. 



I do not think, however, that his proposal to substitute the term 

 " Lower Holaster zone " for the zone of H. subglobosus will meet with 

 acceptance. There are several obvious objections to it which it is 

 hardly necessary to specify, and, in fact, he does not seem satisfied 

 with it himself, for in the table with which his article concludes it 

 is replaced by the designation of "Two Holasters zone." He might 

 at least have made up his mind to use one or the other and not both. 



I am quite prepared to admit that S. subglobosus is not a satisfactoiy 

 index to the zone, but the real question is, can a more satisfactory 

 one be found ? 



This question was briefly discussed in the introductory chapter to 

 the second volume of the Memoir on Cretaceous Eocks, and I wrote 

 that " so far as we know at present there is no species which is 

 restricted to the upper part of the Lower Chalk, and is at the same 

 time common enough in every district to be a useful index." When 

 preparing that memoir I hoped that ITa'ploceras Austeni would prove 

 to be such a fossil, for though it is not very common it would have 

 served well enough if its range had been liu\ited to the zone. 

 Unfortunately, however, near Lewes, in Sussex, it occurs in the 

 zone of A. varians and has not so far been recorded from the higher 

 zone in that county. 



Sharpe, in describing his Ammonites Austeni, states that it is 

 common in the Guildford district of Surrey, but no one has yet 

 ascertained in which zone it there occurs. It is fairly common in 



