508 A. J. Jukes-Browne — Zones of Loicer Chalk. 



Next as regards the zonal position of the Totternhoe Stone. The 

 reasons for placing it in the higher rather than in the lower zone 

 were stronger than he imagines them to have been. If he had 

 I'eferred to my first paper on " The Subdivisions of the Chalk " ^ he 

 vv^ould have found that under the head of Totternhoe Stone a special 

 point was made of the occurrence of a nodule bed at its base, and 

 attention was drawn to certain fossils as characteristic of the stone. 

 The reasons which weighed with me when compiling the General 

 Memoir were the following : — 



1. The frequent signs of current erosion at or near its base, and 

 the change of physical conditions thus indicated. 



2. The px'esence of certain fossils which are generally characteristic 

 of higher beds and are rarely or never present in the Chalk Marl of 

 those counties in which the Totternhoe Stone is typically developed. 

 The chief of these are Acanthoceras rotomagensis, A. cenomanensis, 

 Saploceras Austeni, Actinocdmax lanceolatus, Pecten Beaveri, 

 P. elongatus, P. Jissicosta, Lima echinatn, Enoploclytia Imagei, and 

 more rarely Holaster subglohosus. 



3. The fact that the stone passes upward into the overlying chalk 

 without any sign of physical change. 



Mr Bosworth has a curious metliod of estimating the affinities of 

 a fauna, for he does not deal with the actual fauna of the Totternhoe 

 Stone, but takes the fossil assemblages in the beds below and above 

 it, and calculates how many species of each occur also in the 

 intermediate band. This is certainly not the usual way, and can 

 only be interesting as throwing a kind of sidelight on the subject, 

 and this only when the comparison is fairly made. With this object 

 Reptilia should be excluded, seeing that none occur either in the 

 Chalk Marl or in the Totternhoe Stone of Cambridgeshire. 



When only the same classes of animals are allowed to enter into 

 the comparison the results are different from those given by 

 Mr. Bosworth. The proportion of Chalk Marl species which range 

 up is the same, namely 92 per cent., but the proportion of the species 

 ranging down from the higher beds into the Totternhoe Stone is 33 

 out of 42, i.e., 78 per cent, instead of only 67. 



If, however, the usual and more rational method is employed, and 

 a larger area is included by taking in the lists from the counties of 

 Bedford and Hertford, where the Totternhoe Stone is typically 

 developed, we shall certainly get a more reliable estimate of the 

 relations of the fauna of that stone to those of the beds above and 

 below. Excluding reptiles but including fish the fauna of the 

 Totternhoe Stone (as recorded in the General Memoir) comprises 

 96 species ; of these only 23 range downward into the Chalk Marl 

 of the counties mentioned, while no fewer than 45 range up into 

 the overlying beds-; in other words, twice as many range upward 

 as downward, and the natural conclusion is that the fauna of the 



1 Geol. Mao., Dec. II, Vol. VII (1880), p. 251. 



* This number includes those recorded by Mr. Bosworth, but omits three 

 Cephalopoda which have only been found at Orwell in a quarry which may or may 

 not be above the Totternhoe Stone. 



