574 Correspondence — T. 0. Bosicorfh. 



THE ZONES OF THE LOWER CHALK. 

 Sir, — In the November number, p. 507, Mr. Jukes-Browne 

 comments on my " curious method of estimating the affinities of 

 a fauna." ^ The faunas compared are those of — 



A. The Chalk above the Burwell Eock of Cambridgeshire and 



Suffolk, containing 51 species, 33 of these being common to 

 the Burwell Rock. 



B. The Burwell Eock of Cambridgeshire, with 87 species. 



C. The Chalk Marl of Cambridgeshire, with 25 species, of which 



23 occur in the Burwell Eock. 

 The "more rational" course advocated by Mr. Jukes-Browne is to 

 ascertain whether A or C contains the greater number of species 

 common to B, irrespective of the fact that fauna A is twice as large 

 as fauna C 



By his method, since all three faunas are closely related, the larger 

 of the two faunas A and C is bound to appear the more closely 

 related to B. Therefore I prefer to compare the percentage of forms 

 in A common to B (67 per cent.) with the percentage of forms in C 

 common to B (92 per cent.). 



With Mr. Jukes-Browne's statement that taking a larger area, 

 including Bedford and Hertford, the fauna of the Totternhoe Stone, 

 excluding reptiles, " comprises 96 species, of which only 23 range 

 downward into the Chalk Marl of the counties mentioned," I cannot 

 agree. His own General Memoir records 31 such forms ; and I have 

 already recorded Nautilus elegans in addition to all these. The 

 Chalk Marl of these counties contains 35 species, of which 32 are 

 common to the 'Totternhoe Stone,' i.e. 91 per cent., a figure in 

 close agreement with the percentage quoted for the smaller area. 

 In support of my contention that the zone of Ammonites varinns 

 should include the Totternhoe Stone, I give the subjoined chart, 

 p. 575. Upon this I have plotted most of the species in the Burwell 

 Eock of Cambridgeshire which are not recorded from the Chalk Marl 

 of that county. The species chosen occur in the Lower Chalk of 

 Folkestone and also at some intermediate locality. 

 Forms above the line ABC have not been recorded below the 



'Totternhoe Stone' of the region indicated. 

 Forms below the line ABC are not recorded above the Chalk Marl 



(unless marked *•'). 

 Forms actually upon the line ABC occur in the ' Totternhoe Stone ' 



and also in the Chalk Marl. 

 Thus the line ABC represents the base of the ' Totternhoe Stone.' 

 The straight line A D represents the top of the ' Totternhoe Stone,' 



and also, as I maintain, the top of the zone of Ammonites 



varians. 

 Thus we see that the fauna which occurs above the base of the 

 ' Totternhoe Stone ' in the north, is within the Chalk Marl of the 

 south, and that the transition is gradual. 



But, on the other hand, the line AD separates two different faunas. 



' See my Article "Zones of the Lower Chalk," Geol. Mag., Sept. 1906, p. 412. 



