Corre8ponde7ice — A. J. Jukes-Browne. 431 



between length and breadth in a large and representative series. 

 I very much doubt whether this would be found to be the case. 



Your reviewer tends to beg these questions by treating of the 

 ^^ group of Micraster frcBcursor^\ This phrase would, of course, be 

 a well-recognized one to denote several allied species, among which 

 M. prcecursor is prominent. That may be the sense in which he uses 

 it, but probably it is not so, for in that case his criticism would not be 

 relevant to my remarks, as I only challenged one species, not several, 

 I suspect that the phrase is current, and was used by your reviewer, 

 to designate just the assemblage which Dr. Rowe named M. prcecursor; 

 and that the vague word ' group ' has been added to a term which can 

 only legitimately denote a species owing to a sub-conscious feeling 

 that the assemblage in question is not satisfactory as a species. If so, 

 the use of this expression tends to confirm my views ; but in any case 

 it is in itself so ambiguous that its use without a definition of its 

 scope, for the time being, is to be deprecated. 



R. M. BRrnoNE. 



27 TwYFORD Mansions, W. 

 August 16, 1913. 



THE DIVISION OF THE UPPEK CHALK. 



Sir, — With respect to the scientific points raised by Mr. Brydone, 

 they have really little to do with the division of the Upper Chalk 

 into two stages. He only concerns himself with the line of division 

 between his two zones of Offaster pilula and Actinocamax quadratus. 

 The main question is this — suppose French geologists are right in 

 believing that there are two faunas of stage-value in the comprehensive 

 Senonian of d'Orbigny, where do we find the most convenient plane 

 of division between them ? At present they draw the line at the 

 top of the zone of Marsupites; I gave reasons for drawing it at the 

 top of a higher zone, that of Pldcenticeras hidorsatum and Inoceramus 

 lingua, which though recognized has not yet been fully examined 

 and defined in France. 



This latter zone must be more or less coextensive with Mr. Brydone's 

 zone of Offaster pilula, and if he can substantiate his zone and his 

 upper limit of it throughout the South of England, it should also be 

 applicable to the Paris Basin, and may eventually become the plane 

 of division between a restricted Senonian and a Campanian stage, as 

 suggested by me last year : that is the real point which requires 

 further investigation. 



Meanwhile I am quite prepared to agree with Mr. Brydone that 

 the Yorkshire Upper Chalk is so decidedly North German in its 

 affinities that its nomenclature should be North German rather than 

 Anglo-Parisian. Let the discussion of the subject be limited at 

 present to the Anglo-Parisian region, but here a caveat must be 

 entered. It is well known that the species which go by the names 

 of A. graymlatus and A. quadratus are connected by a number of 

 intermediate forms, and that Mr. Rowe regards the one as the lineal 

 ancestor of the other. Mr. Brydone will have to define exactly what 

 he means by A. quadratus and what he regards as the distinction 



